How fun! I love a themed party! Probably depends a bit on who is going and how dressy it is and if it's actually near a beach. If you're completely in the dark and know someone else who's going, I'd definitely recommend checking with them because it's possible people just show up in a bathing suit with a coverup. If you know it's more an outfit theme than an actual beach party, I'd suggest what the other commenter did and go with a mermaid inspired look.

If channeling Ariel, a long tight green (or almost any shiny) skirt and purple crop top is pretty easy to find and pull together. If you want more flexibility but still want to be thematic, any outfit with fishnets could probably have a similarly colored fish. Black, white, and orange plus fishnets and all the sudden you've caught Nemo. Blue and yellow could be dori. It also works with fish that aren't animated characters, but it might be more fun if you have a reference point and people might be able to guess it. You could probably find a cute outfit you already have and like with fishnets and search "[outfit colors] fish" and just be done there. They also make net tops, but I'm not sure how practical that is as a wardrobe piece outside of this for you. Alternatively, anything blue, white, or tan would still be on theme. If covering your hairline is a must, maybe a thick green headband. Could be seaweed inspired.

If you don't mind being the odd one out, you could go with a sailor/fisher inspired look and those have built in hat options and can be a little camp if you've got the personality to pull it off. Pirate is a similar option but may be even more removed from the theme.

Good luck! Would love to know what you end up going with!