Probability of this happening very often is gotta be uber low, but I had a strap dangling off the back of my round boat on the Rio Grande outside of Creede and the passenger in the back of my boat informed me of a snake that had climbed up said strap on it's way into the boat. Luckily he was not afraid of snakes, and removed the snake.
As far as the thread topic, I see entanglement risk being low vs being an aid in potential swim saving yourself situation. Probably wouldn't go to the effort to set them up unless in big or super technical water.
“HOLD THE DOOR!” — Hodor