Where do River Otters sleep?

Answer 1

River otters sleep in their holt's (dens), which can be found under tree roots along river banks or in dense undergrowth adjacent to rivers. Natural holt's are becoming pretty rare nowadays, due to the removal of old riverbank trees; however conservation groups are providing artificial holt's in strategic places to allow their natural habitat to recover. Otters may have several holt's a long the river bank, which provide shelter when out feeding and patrolling their territories.

Answer 2

They usually find reeds and wrap in that so they don't sink or float away then go to sleep on their backs. So I guess to answer your question, water.