Otter are like beaver, they may float or sink if killed instantly with a head shot. If they don't die instantly they will sink. Rats float better than most critters, and if you are ever invaded by sea otters or spotted seals, they also need to be killed instantly or will sink.
Otter travel a long circuit when hunting and travel through rather quickly unless, as some of you found out, they find a well appointed buffet. They are difficult to trap using bait and the best otter sets are blind sets where they cross over a dam, have a slide down a bank or have their travel coridor restricted such as a narrow creek.
