Ok, quick rule that can help you, don't cast spells for trivial matters. God's/universe/whatever you work with, never favours people who use magick as the cheat code of life. Casting one spell to help when in need is fine, casting a spell for something you can do yourself isn't wise.
I'm guessing you want to become a mermaid [which you have discovered doesn't happen with real magick] you cannot cast a spell and be able to hold your breath for long periods either. You could use a chant to help you while practicing holding your breath as a way to empower you and help work towards your goal. Humans can train themselves to hold their breath for longer periods. Two things I think the longest on record is 5 minutes, and our bodies cannot handle the pressure under so many feet of water, so holding your breath for long periods is a good skill to learn if you plan to swim for long periods, you might want to take scuba lessons. [Plus there's a company that makes mermaid tails you could consider buying one if you really want to pretend to be a mermaid. Could also make some money posing with tourists if possible]