I agree, the merganser were always one of my favorites, like you said, they are just always getting into stuff seems like. The goldeneyes are nice too and have an awesome courtship display, either work great (buffleheads too, they are more less a tiny goldeneye.
Common goldeneye arent aggressive at all, they'll bit a tail here in there if they run over them, the Barrow's are the more grumpy ones, though I never had an issue with them either in large aviaries. Any will be good, just remember they all need fish food. Divers have a higher protein intake in the wild than puddle ducks, so it 's really not going to matter unless you go with pochard/red head style ones. They dont need it, but it helps.
All the saw bills, goldeneye, sea ducks, etc, they all need the fish food. You dont have to feed any of the fish, but it's fun to watch if you do, especially if you have crystal clear ponds. I use to have minnow traps out in our pond just for that reason. I'd give them anything I caught, from small fish to crawfish, tadpoles, small eels, anything. They look like otters under water chasing them down..pretty cool to watch.
jgell, there are breeders scattered all over the country. The Great Lakes area is crawling with them. Lonnie Bougie in Green Bay has just about everything, especially diver related and has awesome birds. He's usually in the Gazette and A.P.W.S. magazines
