I'd start here: http://brickset.com/sets/tag-Floats-On-Water
No experience with boats but with LEGO RC cars, I would use 2 elongated floating hulls to make a sort of a very stable catamaran and as @Jenni mentioned, paddle wheel on each side, both motorized by part 5292 (Buggy motor). I have tried all kind of motors, and it is the most powerful. SBrick so you can still control it when it is more than 10' away...This page is good to give you detailed analysis of every existing LEGO motor: http://www.philohome.com/motors/motorcomp.htm
It looks like you're new here. If you want to get involved, click one of these buttons!