Otters tend to clear out once they realize people are around, at least around here. I do not recall having read nor hear of a problem between otters and dogs while hunting around water. If an otter is caught on land, that is a different matter. My dogs have faced off with otters out in the fields on two occasions but luckily it was a stand off until I could get leads on my dogs. An otter at 10' can sound pretty ferocious and a family of four is downright scary.
Beavers are a little different. They are territorial of their lodge so will defend it if they feel threatened. Every year there are reports in the paper of people being attacked by beavers while swimming. Even so, I don't think the problem is very serious as many of the stories involve late night and alcohol which is not typical while duck hunting. That said, I probably would not use an active lodge as a blind site as that might cause a reaction from the residents inside. I've done so with muskrat lodges and have had the owners come topside voice their displeasure. A three pound rodent cussing me out is not as attention getting as a fifty pounder.
My main concern if beavers are around would be my dog getting impaled, cut, or snagged on the various branches stuck into the bottom. Rationally, the risk is probably only a little greater than the same thing happening on land but my ability to aid my dog is less which increases my concern on the emotional side of the equation.
