Borsalino is my favorite marine along with Garp. And i've seen a lot of people give this arc shit and i kinda understand why, main antagonist of this arc is depth-less but i see this arc more of a backstory of fishmen and why they hate humans and vice versa so if you look at it like that it's ain't that bad
Now thats just sad, selling out the one who brought back your daughter... hell, saved her in exchange for her not being taken back by the fucked up marines. Tragic, but a hard decision.