Broadheads are a must with lighter bows. You can kill them with other points, but why risk wounding them for no real reason. Heavier bow/arrow combinations will have better results with tips other than broadheads.
If dogs are involved, broadheads make me nervous. If they are steady or tied, I would be less concerned. Dogs are a great asset for catching/finding birds that get "clipped".
Two blade broadheads are easier to wipe off and resharpen.
Nothing wrong with shooting them on the water. The are still a tough targets. Beware, they can get airborne pretty quick.
If you want the ultimate in sporting, shoot them in the air. Just to clarify, as soon as their webbed feet leave the water, they are in the air and try to hit them fast. The more time they have in the air the harder it's going to be to hit. Once they get going, they become extremley agile. On multiple occasions, we have witnessed them literally dodge the incoming arrow. I vividly remember one duck last season. It was coming over our heads and my brother took the shot. It was perfectly timed. At the last milesecond, the drake rolled over like a fighter jet dodging a missile, and continue on unharmed.
They are a lot of fun to hunt and you can get a lot of shooting. Have fun!