Since seasickness is a totally different thing from swimming/diving, I can buy an Illya at home IN the water, just not so much ON it. :-)

As with sailors who can't swim (possibly not so true these days, but historically), I can sort of buy a sailor who hates water, but I don't buy that for Napoleon. That's too obvious a weakness for a spy (not being able to swim, I mean). Even if RV felt that way, Napoleon would've had to overcome it.

So I see them both as capable on and in the water, even if it isn't their very favorite pastime.