Here in this thread I would like to talk about the mighty King of the sea, Poseidon. I will be showing the many stories about him and I will list certain offerings you could give him afterwords.
Lets start from the beginning...
Because of the prediction of the downfall of his reign as king, Cronus began eating his children in fear one of them would do this. Zeus, being the lucky one, was replaced with a rock by his loving mother so he wouldnt get eaten. When Zeus was old enough he fed his father a strange concoction, which made him throw up his still living children that was in his stomach. This was later followed up to be the overthrow of the titans and the age of the Gods.
According to how the myth of the birth of Poseidon began, he was conceived from his mother Rhea and then was later eaten by his father Cronus, who were both Titans. In some stories, just like Zeus, he was hidden in a different place to be raised by another group of people, called the Telchines. In this story he would have fallen in love with one of them, named Alia, and later would have children. She would have gave birth to seven children, six boys and one girl. The girl's name later became the name of this island, which was Rhodes.
During the war with the titans, some titans were on the side of the Gods and later set free a group of three brothers called the elder cyclopes. They were so happy that this happened they crafted three magical items that were given to Zeus, Poseidon, and there other brother Hades. Poseidon was granted the trident, which had the power to make earthquakes and was later made to control the seas.
Later on after the downfall of the Titans, Zeus, Poseidon, and Hades, drew lots to see where they will rule. Zeus being the more fortunate got the skies and became king of the Gods, Poseidon got all bodies of water and became ruler of the seas, and Hades, being less fortunate, became the ruler of the underworld.
In many myths Poseidon, like Zeus, had many love affairs from his wife. One being so with his own sister Demeter. One day when Demeter was wailing in her own despair from the loss of her own daughter Persephone, Poseidon pursued her. Feeling offended Demeter took a form of a horse and hid amongst a herd. When Poseidon heard of this he took a form of a stallion and raped her. Thus allowing her to give birth to talking horse Areion and the Goddess Despoine.
Another tale, which I prefer, is the creation of the horse. In this tale he flirted with Demeter, but she showed him no interest. In reaction to this he tried to create an animal so perfect that she will fall in love with him. After many tries and creations of many animals, he created a horse, but later grew less interest in her.
During another a myth, there laid a nameless city with a king looking for a patron God to watch over. This king was named Cecrops and he noticed that two Gods wanted the city named after them so badly that they ended up fighting about it. These Gods were of course the mighty Poseidon and the wise Athena.
Cecrops couldn't decide so he allowed a contest to who ever could give a better gift for the people of the city will win. Sadly Poesidon's gift was not as wonderful as he thought it would be. He struck down his trident and a spring salty water sprung forth. This did not please the people of this nameless city. Athena though planted an olive tree which could be used for oil, wood, and many other things. Thus, being said Athena won and the city was named Athens, which made Poseidon mad.
In the tale, the Odyssey, Poseidon is the antagonist. But in my own opinion he was just looking to seek justice after Odysseus blinds one of his own sons. Poseidon, being the powerful God he is, struck storms and high winds to prevent him going home in Ithaca. As the story continues his boat sinks and his whole crew dies from drowning, thus leaving him alone in the middle of the ocean.
What made Poseidon even angrier was that he was on the side of the Trojans and Odysseus was on the side of the Greeks. This made him furious when the Greeks took over Troy, which kindled even more hatred for Odysseus.
The story of how Poseidon and Amphitrite got married is quite simple to remember. Amphitrite is one of fifty Nereid who are the daughters of Nereus and Doris. Poseidon became desperately in love with her as soon as he spotted her dancing and he came up to her and ask her to marry him. At first she said no, but Poseidon couldnt take no for an answer and sent his dolphin helper to plead his cause. The dolphin pleaded so well she finally said yes. Among their children were the half man and half fish god Triton and there daughter Rhodes. (Rhodes has different variations of stories who her mother was.)
Now we will be discussing certain offerings that you can give to Poseidon...
Myrrh is used in the Orphic hymns to Poseidon, but in my own opinion frankincense is a good all purpose incense given to any God of the Greek pantheon as well.
The ancient Greeks for some reasons associated pine trees with Poseidon, so anything from pine needles to pine cones could work as a great offering to him.
A deep blue candle can work as an offering to him as well. It represent the deepness of the ocean and can be lit in his honor.
Fish is a great example as an offering to Poseidon.
I gave Poseidon a wooden boat, which he loves, so anything nautical would be a good choice as an offering to him.
Milk and honey were given to any God in the Greek pantheon as a choice for a libation.
Seashells or anything beachy.
Write some poetry about him or make your own hymns.
Dedicate a day of a week where you do a special ritual for him.
Whiskey is usually cracked on a ship before they take off to the sea for the first time for good luck, I couldnt see how why Poseidon wouldnt like this.
I hope you liked this thread and I will try my best to make more on the topic of the Sea Gods of the Greek pantheon and hopefully some on sea witchery
In the interest of fairness and presenting both sides I would like to give some historical text on Poseidon and his sons. Keeping in mind I will be linking my credible sources at the bottom for anyone who would like to read further into the subject. As a disclaimer I would like to state everyone is perfectly entitled to honor whatever deity they like and interpret things however they want. But they should be given both sides and then be allowed to form their own conclusions.
"ARES WRATH : HALIRRHOTHIUS
LOCALE : Athens, Attika (Southern Greece)
Pseudo-Apollodorus, Bibliotheca 3. 180 (trans. Aldrich) (Greek mythographer C2nd A.D.) :
"Agraulos [daughter of Kekrops king of Athens] and Ares had a daughter Alkippe. As Halirrhothios, son of Poseidon and a nymphe named Eurtye, was trying to rape Alkippe, Ares caught him at it and slew him. Poseidon had Ares tried on the Areopagos with the twelve gods presiding. Ares was acquitted."
Pausanias, Description of Greece 1. 21. 4 (trans. Jones) (Greek travelogue C2nd A.D.) :
"There is a spring [near the Akropolis, Athens] , by which they say that Poseidon's son Halirrhothios deflowered Alkippe the daughter of Ares, who killed the ravisher and was the first to be put on his trial for the shedding of blood."
Pausanias, Description of Greece 1. 28. 5 :
"There is also the Areopagos (Hill of Ares) [at Athens] , so named because Ares was the first to be tried here; my narrative has already told that he killed Halirrhothios, and what were his grounds for this act."
Seneca, Hercules Furens 1341 (trans. Miller) (Roman tragedy C1st A.D.) :
"My land [Athens] awaits thee. There Gradivus [Ares] once cleansed his hands from blood [i.e. for the murder of Hallirhothios] and gave them back to war."
"In a late version of the Medusa myth, related by the Roman poet Ovid ( Metamorphoses 4.770), Medusa was originally a ravishingly beautiful maiden, "the jealous aspiration of many suitors," but because Poseidon had raped her in Athena's temple, the enraged Athena transformed Medusa's beautiful hair to serpents and made her face so terrible to behold that the mere sight of it would turn onlookers to stone.  In Ovid's telling, Perseus describes Medusa's punishment by Minerva (Athena) as just and well earned."
Link: (Please keep in mind whether or not there was consent involved difers since there is more than one version. Wikipedia does actually cite their sources, this is a legitimate alternate version.)
" Aloadai ) were Otus (or Otos ) and Ephialtes , sons of Iphimedia, wife of Aloeus, by Poseidon,  whom she induced to make her pregnant by going to the seashore and disporting herself in the surf or scooping seawater into her bosom.  From Aloeus they received their patronymic, the Aloadae. They were strong and aggressive giants, growing by nine fingers every month  nine fathoms tall at age nine, and only outshone in beauty by Orion.  
The brothers wanted to storm Mt. Olympus and gain Artemis for Otus and Hera for Ephialtes. Their plan, or construction, of a pile of mountains atop which they would confront the gods is described differently according to the author (including Homer, Virgil, and Ovid), and occasionally changed by translators. Mount Olympus is usually said to be on the bottom mountain, with Mounts Ossa and Pelion upon Ossa as second and third, either respectively or vice versa. Homer says they were killed by Apollo before they had any beards,  consistent with their being bound to columns in the Underworld by snakes, with the nymph of the Styx in the form of an owl over them. 
According to another version of their struggle against the Olympians, alluded to so briefly  that it must have been already familiar to the epic's hearers, they managed to kidnap Ares and hold him in a bronze jar, a storage pithos , for thirteen months, a lunar year. "And that would have been the end of Ares and his appetite for war, if the beautiful Eriboea, the young giants' stepmother, had not told Hermes what they had done," Dione related ( Iliad 5.385391). He was only released when Artemis offered herself to Otus. This made Ephialtes envious and the pair fought. [ citation needed ] Artemis changed herself into a doe and jumped between them. The Aloadae, not wanting her to get away, threw their spears and simultaneously killed each other.   "
"After having raped Caeneus, Poseidon fulfilled her request and changed her into a male warrior."
I recently stumbled upon another one I would like to add.
"Ovid, Metamorphoses 4. 770 ff (trans. Melville) (Roman epic C1st B.C. to C1st A.D.) :
" [Medousa (Medusa)] was violated in Minerva's [Athena's] shrine by the Lord of the Sea ( Rector Pelagi ) [Poseidon] . Jove's [Zeus'] daughter turned away and covered with her shield her virgin's eyes. And then for fitting punishment transformed the Gorgo's lovely hair to loathsome snakes."