I slightly disagree and have killed numerous divers from my float boat although it can be difficult. Ive shot Buffies, Scoter, Broadbills, Whistlers, Oldsqaw, Eider, Mergansers,and Redheads from my boat, some near shore and some offshore. I too have gone to using longline rigs for easy setting and pick up. Many times divers prefer their own species so I tend to set them in separate groups but it often doesn't matter as they will generally give your rig a swing even if they don't commit. Puddle ducks are are another matter. Although you can kill Blacks or Mallards near shore if your boat is well camouflaged. Being wary, they are difficult to entice offshore as the boat stands out. They will generally land outside your set, if at all, although they may swim into your decoys. You can't kill them on the couch.
Good luck!
Tomorrow I'm headed to Western NY for a few days for my last ducks of the year.