A good majority of boat hulls have forming or bends as part of their constrution, and are made from 5052, or in the case of some of the higher end makes, 5086 (more corrosion resistant). 6061 is used more for bar or extruded shapes, such as channels, tube, angle, even custom shapes like chine and gunnel forms. 6061 does not like sharp bends, and it also looses quite a bit of it's strength in heat affected weld zones. Part of the reason 6061 chine and gunnel extrusions have become so popular, are you get a good mechanical fit-up without having to be precise on the joint cuts (the sheets fits into a groove), the welds are usually on the inside, giving a nice trimmed out look on the outside, and the mechanical joint helps offset the loss of strength from welding.
I have noticed a newer line of boats from Smokercraft called "American Angler", and I believe these are all welded.