You can not psychically become a mermaid on this plane/earth. it is impossible to do that. But when you are on the astral planes, your welcome to change into any form you wish. Because it is your spirit/soul. Changing your psychical body would be painful.
None of the mermaid spells on here work, they are fake, far as i know. Even all vampire,werewolf spells are fake, Mainly those are just fluff or role players who came on here and posted those.
But yes as long as your keep practicing your basics, you will get better at time, believe me, i am starting to understand the occult a little everyday more and more just by studying and practicing what i can. Meditation is your best bet to start out with, It will help you focus and gain control of your energies.