Nathan is useless. He can swim well enough and float on things well enough to not drown, probably unless he stops caring, but he's not going to be able to help anyone. I mean, he could try, I guess, but then everyone would drown, so.
Chekov (the version I'm playing, anyway) had a traumatic near-drowning experience once and is a pretty terrible swimmer. THAT SAID, he'll try his very hardest to be helpful. Completely game for having him team up with folks making elaborate plans or building elaborate rafts!