GM Performance Parts (while not a marine motor), has some strong motors, and the LS small blocks are already in the boating market (not on a mass produced schedule) with success (examples: Jim Kid's 30' Skater two LS7 small blocks and Weisman's race boat with LS small blocks, are two which I can cite).
I have Dart 540's in the Phantom, and yes, they are a great block.