Even if he did drown, his body and gear ought to be found where he did croak, as UrW doesn't (yet) account for whether things ought to float or sink. I've never seen a Njerp swim, however, as they stop their chase when the water gets too deep to reach the bottom.
I did once encounter a Njerp on a 3*3 skerry 5 or so tiles off the shore. As the skerry wasn't climbable, I couldn't collect his gear and some of my arrows, though, as he died at the center. I suspect he spawned there. This was a fair while ago, before the magic system rewrite.