I wouldn't poo in the sea because I'd rather do it in a toilet with a shatafa.
If you're grossed out by the idea of taking a crap in the sea, but you'd happily shit in a toilet, wipe your bum with a tissue, & go about your business without washing your nethers properly, with water, after a poo...
...then quite a lot of people, if they knew about it, would think you're a weapon's grade minger for not bideting your bum.
The fish won't care (except they probably like poo crumbs).
Probably best not to over think it, & just make sure your own bits are as clean as you'd normally like.