Right in our backyard:  Otters!

From the "Living with Wildlife" page on the Washington State Fish and Game site - I learned that these are "River Otters" and that they are quite common in Western Washington:  

"Although seldom seen, river otters are relatively common throughout Washington in ponds, lakes, rivers, sloughs, estuaries, bays, and in open waters along the coast. In colder locations, otters frequent areas that remain ice-free in winter—rapids, the outflows of lakes, and waterfalls. River otters avoid polluted waterways, but will seek out a concentrated food source upstream in urban areas."

The Fish and Game web site goes on to say:  "River otters are active day and night; around humans they tend to be more nocturnal. Otters spend their time feeding and at what appears to be group play. They also dry their fur, groom themselves, and mark their territory by vigorously scratching, rubbing, and rolling on the ground. River otters are active year round, and, except for females with young in a den, are constantly on the move. They tend to follow a regular circuit that is covered in one to four weeks. Males can travel 150 miles within a particular watershed and its tributaries in a year. A family may range 10 to 25 miles in a season.

To observe river otters, sit quietly on a high place (a bridge, overhanging bank or tree, or pier) above a known feeding area, trail, or slide. Find an angle from which you can avoid surface glare. A pair of polarized sunglasses and binoculars is useful. River otters are wary and their hearing and sense of smell are well developed. However, they are fairly nearsighted and they may not notice you if you stay still.

Never instigate a close encounter with river otters. They have been known to attack humans, and females with young are unpredictable."  

The otters moved very fast - ducking below the surface of the water in the blink of an eye; only to reappear 20 feet away.  It was hard to tell if all the bubbles on the water surface were from the swimming activity or from the otters exhaling under water.