He basically wanted as little to do with 'normal' humans as possible - he really meant it when he said he'd be gone in an hour. Also, remember his explanation as to why he didn't take Helen up on her offer the first time - it was because he knew she didn't really want to. In other words, he wasn't into having sex with someone who was being forced into it. The scene at the beginning was the first hint that he had a 'softer' side to him.

His fish smell down there is worse than the lady's.

You mean otherwise they would have forced him to give someone good (fish) head? Yeah, i imagine he wanted to avoid such

He didn't want them to notice the small vagina he had behind each ear.

He also resisted the naked Jeanne Tripplehorn(y), which - I imagine - is pretty hard (ask Mike Douglas)
.
So, since the mariner is a mutant he might as well have fish-genitals, which might make it practically impossible for him to have sexual intercourse with a pure humanoid female.
And therefore, he also might only got turned on by a special kind of (probably female) fish, perhaps a nice and curvy, slippery and wet blowfish
Oooooorwell, maybe the mariner himself was feeling embarassed, because someone coulde have seen (or smell) his 'fishy' weener, who knows?
PS.English is not my native tongue. Just if some things sound funny to you

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He prefers his ladies leave their eggs in the water for him to fertilise with a cloud of his..well, you know.
then whoa, differences

Exactly he didn't want anyone close to him she would have noticed that he's a mutant and would have told the others about it.

He is just gay.

I assume you mean the girl he was "offered", right before the tried to leave. He probably didn't want them to know
he's a mutant.
Considering how it turned out I can't say I blame him to be on the cautious side.