River otters spend two-thirds of their time on land. They will make at least one permanent den on land, near the water, in addition to several temporary shelters. They are adept at being on land unlike the clumsy and rather awkward moving sea otter, who seldom spends 95% of their time in the water.
Sea otters are active during the day while river otters are nocturnal. River otters are often solitary creatures. When they are seen together, it would likely be a family unit such as a mother with her young.
The 1980's were hard on populations due to oil spills. The 1989 Exxon Valdez oil spill in particular, in Alaska’s Prince William Sound, killed several thousand sea otters. The oil ruins their fur's ability to keep warm enough in the frigid waters, as well as being toxic. Many died from hypothermia. Estimates put the current worldwide population at only around 106,000.
In addition to oil spills, there are other threats to sea otters.
Sea otters are often contaminated with toxic pollutants and disease-causing parasites as a result of runoff in coastal waters. "Scientists have also reported the accumulation of man-made chemicals, such as PCBs and PBDEs, at some of the highest levels ever seen in marine mammals." ~ Defenders of Wildlife
They are one of the few mammals who use tools. They will use a rock to break open a shell or use something to pry it open.