.22 rim-fire would work better. It would also be safer for the swimmers ;-)
Good solution my method is minimum time investment. You could trap shallower and shoot when caught. Possible balance point in investment of time.
22 rimfire would be safer for the swimmers, but good luck getting one in the heart lungs or brain of an adult beaver. Their skin is pretty tough. Enough in the body might get you an eventual kill but I prefer a clean kill . . .
We cap em with .223 in Ga - no season or limit. Once through the lungs and they are pretty much done.