I too have been wondering about having a duck buddy to take kayaking, except I'm a whitewater boater. I've seen plenty of wild ducks, geese, and cormorants go through class II rapids, maybe even an easy class III but certainly never a class IV or V (although I did see some panicked goose parents at the top and bottom of a four foot waterfall made by an old dam, waiting to save their wayward gosling. It caught the last eddy at the top fortunately, or the hydraulic at the bottom would have been pretty rough on it, though my friend who's an excellent gnar boater was prepared to save it). Waterfowl always seem to know where to go on the rivers and creeks. THEY never get jammed up in shallow areas or pick the channel that turns out to be impassable. My only duck that is attached enough to me to take out is a Muscovy though, so I don't think rivers are his thing. Maybe one day I'll have a water duck imprinted enough on me to take on Antietam Creek, which is class II- under normal conditions. But only if I'm 1000% sure it'll stick with me.
I don't know from sea kayaking, but I would think the big waves on a lake would make it easier for you to get separated from your duck. Even if he normally comes when you call him, he might not be able to hear you or might panic and fly off. Or find the duck mate of his dreams. With trimmed wings, he might feel all out of whack on the water. And the predator issues that others mention would certainly be a concern. I would err on the side of caution and stick to the familiar creek.
I don't know from sea kayaking, but I would think the big waves on a lake would make it easier for you to get separated from your duck. Even if he normally comes when you call him, he might not be able to hear you or might panic and fly off. Or find the duck mate of his dreams. With trimmed wings, he might feel all out of whack on the water. And the predator issues that others mention would certainly be a concern. I would err on the side of caution and stick to the familiar creek.