While it may appear to be adorable, sea otters hold hands for a practical purpose. The behavior known as 'rafting' is where single sex groups of sea otters numbering anywhere from two to several hundred group together, often holding hands to prevent themselves from drifting apart.
Rafting is part of the sea otter's social behavior, but it is also part of the animal's general practice of attempting to stay stationary while floating on their backs in the sea. These animals have been observed taking other measures to stay stationary while floating, including wrapping themselves in kelp.
Though they are amphibious, sea otters perform a number of essential behaviors while floating on their backs, including sleeping, resting and eating.
There is no evidence that hand holding in sea otters is a display of affection in the way it is with human beings. Though otters are social creatures, they can be quite violent with each other. During mating, a male sea otter will bite down on his female mate's nose, which typically leaves the female's snout bloodied and will usually leave scars. However, otters do engage in a variety of other behaviors that humans tend to find endearing and charming, including a behavior in which a sea otter mother will float on her back with her pup balanced on her chest.