My faves are Royal Robbins Backcountry shorts. You can't truthfully describe them as waterproof but the synthetic material they're made from is so light and quick drying they're the next best thing. About £35 a pair, I think they're intended for go anywhere, do anything type of wear in warm climates, and if that includes a quick dip in the sea or jumping in a river, then that's fine. They are so light and comfortable to wear, you hardly know you've got them on, and I like them so much I've got three pairs.