The only girl
in an underwater kingdom who can’t swim in an underwater kingdom must cross a dark wasteland to prevent the destruction of her home #PitMad #YAFantasy
This has promise! The bit in red is a necessary grammatical change (the original phrasing means that she is the only female in an underwater kingdom, and the kingdom itself can't swim). The bit in bold I felt could maybe be made stronger - especially 'cross,' because I sort of just envision her crossing the wasteland like she's crossing a street, ha. It should definitely feel more dramatic than that. I feel like overall it's not quite specific enough.
18yo Ondine rides manta rays, fights
eels and sharks, and matches wits with a giant, telepathic squid to save her home #PitMad #YAFantasy
I like this one a lot more, and it also strikes a very different tone. This builds a rich setting and feels fun and fresh. The words I crossed out were just places where I thought *maybe* you could use the extra characters to work in that she's an underwater princess with an amputation, but it might be too much to squeeze in. But if you could give her a quality more distinctive than just '18 years-old' I think that'd make this memorable (sassy, timid, daring, clever, determined? Something zippy and not cliche. If you had to describe Ondine in just what word, what word comes to mind?). Also: maybe you can say 'kingdom' instead of home, making it clear she lives in the ocean and not, say, Orlando, Florida.
2 exiles meet on the ocean floor; one a princess who wants to clear her name, the other a merchant seeking to betray her #PitMad #YAFantasy
This one has a nice rhythm to it. I think I'm leaning toward the second one out of the three because it packs in more details (a rich setting). But this one might make an agent feel you're a good writer and get bites for that reason.
Also, maybe you can say 'disabled' if 'amputee' doesn't feel right, because that might stir interest. It's definitely not your average YA mermaid book.
So I rank them in this order: 2, 3, 1. I hope this helps! I wouldn't be surprised if you got bites on these, especially the last two.