Sea otters sometimes hold hands while they're sleeping to keep from floating away from each other. Often a mother and pup will hold on to each other while they are resting so that they don't drift off from each other or the rest of their group, which is called a raft.
Sea otters holding hands isn't that common in the wild, according to the Seattle Aquarium. Instead, large rafts are more likely to keep track of each other by keeping an eye on one another, listening, keeping in touch through occasional tail and flipper contact. "Paw holding is most likely a learned behavior specific to certain individual otters, who may find it comforting!" according to the aquarium.
In addition to holding hands, sea otters also often sleep on their backs, wrapped in long strands of kelp which grow from the seafloor all the way up to the surface of the water. They use the kelp as an anchor while sleeping so they can nap without the worry of floating out to open ocean.