Hmm, I might answer it as nobody else wants to, despite knowing it beforehand; it's a popular physics puzzle.
Answer is that the level falls (or stays the same in a corner case). The reasoning is as follows. If he's in the boat, his weight displaces water (putting this water above the regular level) equal to his weight (pressure), whereas if he's in the lake, he displaces water equal to his volume. The water amount equalling his weight is more than his volume; if it were not, he wouldn't have drowned, but floated.
The corner case happens with exact same densities, in which case inertia from falling would have made him drown.
-- Simon