I have to say this part about beavers seems entirely made up. Beavers do not slap their tail as an aggressive act, they slap their tail to warn the other beavers there is danger and that they should hide. They do it at the first sign of trouble. There is no way you would be able to even get in the water without them noticing you first, and beavers never attack people.
Also beavers will be found in the ponds they create with dams in streams found flowing into the rivers. A beaver pond wouldn't likely be a very enjoyable place for humans to swim, you would be risking catching beaver fever (Giardiasis).