By the way, do you also usually call otters "he" by default, until you know? I don't want to call them 'it'. Maybe they should be called otter.

I can't really tell the difference, but I struggle with the captcha so I'm not a good example
I usually go with singular they when I don't know. I know some people don't like singular they, but "you" is also a plural pronoun that is used in singular form and I think if I insisted people use "thou", the singular version, they'd pop a gasket so it all seems kindof silly to me.

Yeah from my experience it's hard to tell between the genders, atleast for me. I suppose the females a bit smaller, but otherwise they seem pretty much the same. I think the males have a bit rougher facial features and broader muzzles, and while I like to believe I can see the difference I doubt I would pass any test! The more obvious clues are ofc the males having their scrotum and the females their teats. Also, female sea otters can sometimes be seen with a scratch on their nose after mating.