In summer, you might see a cool Osprey over the lake, but the waterfowl are pretty scarce. Winter is the hot time for animals on the water. By November, pretty much any duck or vaguely duck-like birds you see on the water is a wintering species here, so the *individuals* might be passing through, since most have a winter range all along the west coast, but birds of those species should be here until March or April. One possible exception would be the Redhead duck, which normally winters farther south, but can be seen on the lake rarely. Most of these species have migrated down from much farther north, but lots of the grebes migrate more east-west, breeding on lakes in the interior US or Canada.
There are a few beavers, muskrats, nutria and otters on the lake, too, but ducks and duck-like birds have the advantage of being out in the open pretty much all the time, big enough to see easily without binoculars, and generally easily identified to species due to their obvious markings (at least the males, females of dabbling ducks are a bit more cryptic and some of the loons and grebes look alike). Knowing what critters are here should make paddling on the lake more interesting. A quick outline:
"Diving ducks" are strong swimmers and can go far underwater to find food. They tend to eat mostly animals, including small fish, insects and even molluscs, depending on the species. They take flight like an airplane: they need a running start across the water and then climb away from the surface gradually, so you're unlikely to see them on small ponds. You're likely to see: Buffleheads, Mergansers (Common and Hooded are easy to ID, Red-Breasted look pretty similar to Commons but are rare), and Scaup (good luck telling Greater from Lesser Scaup!) and less often Goldeneyes (Barrows and Common), Scoters, Canvasbacks and Ruddy Ducks. Ringed-Neck Ducks look a lot like Scaup but have a white ring near the tip of their bill. It's hard to see any ring around their necks, ironically. Buffleheads are small, Hoodies and Scaup are a little smaller than Mallards and Common Mergansers are big, for a duck.
"Dabbling ducks" are more buoyant than diving ducks and can't really swim effectively below the surface, so they mainly eat things they can reach by tipping their tails up and reaching their heads down a few inches. They tend to eat more plants than diving ducks, but will take insects and snails. They can leap almost vertically into instant flight from land or water and climb steeply upward, so they can be found on land and small ponds as well as the lake, and are sometimes called "puddle ducks." Most common are Mallards, Widgeon. Shovelers, (which look kinda like Mallards with huge bills), Green-winged Teal (small) and Gadwalls. Gadwalls are mostly gray, and would be boring except that you may see them stealing food from Coots (see below), which unlike the dabbling ducks can dive at least far enough to pull plants from a few feet down on the lake bottom. Less common but cool-looking are Wood Ducks and Pintails. Canada Geese act a lot like dabbling ducks, but I suppose everyone is bored with them since they swarm everywhere in parks, pooping constantly. If you see any other type of goose or a swan on Union Bay, it's your lucky day. Go buy a lottery ticket immediately!
The most common non-duck on the lake is the Coot, a drab dark gray critter with weird toes. It's actually a type of rail, a fairly obscure family of birds from which the phrase "as thin as a rail" comes, but coots aren't thin, they're box-shaped. If you're lucky, you may see a Bald Eagle stampeding the coots across the water, but I've never seen one of them actually get caught by the eagle.
Double-crested Cormorants are also around pretty much all the time. Unlike any of the other waterfowl around here, their feathers absorb water, so they are heavy fliers, needing a long run to get off the water, and often sit with their wings half-spread trying to dry off. They're related to pelicans, and dive well to catch fish. They live in the trees along the Montlake Cut.
You may see several types of grebes. All the grebes are strong underwater swimmers who eat fish, can barely walk on land and will usually dive when you approach, rather than fly away. Pied-billed Grebes are here year-round, and are the smallest common grebe (smaller than any of the ducks). Red-necked Grebes are fairly common in winter, and look kinda like a loon except that they are smaller, about the size of a medium duck. Horned Grebes and Eared Grebes are smaller, rarer and not too exciting in their winter plumage. Western Grebes are big and more strikingly colored than the other wintering grebes, but you rarely see them in Union Bay or Lake Union. If you go right out in the middle of Lake Washington or well away from shore on the Sound there may be large flocks of Western Grebes.
Loons are like grebes ^2. They're big, strong divers, who can stay underwater so long that after you see one dive, you'll probably get bored and move on before it comes up. They can often be seen swimming along while sticking their faces underwater intermittently, and if they see a small fish down there, it's probably all over for the fish. Unfortunately, their winter appearance is not as striking as their breeding plumage, and they're not very numerous near the WAC. Loons on the lake are usually Common Loons.
Except for collecting sunlight for Vitamin D, winter is the best time of year to go out on the lake. In summer, it's usually too calm to be interesting, and there are almost no easily-seen animals except a few Mallards, Crows, Red-winged Blackbirds, maybe an Osprey and those resident Bald Eagles. OK, Blue Herons too, which are cool. But in winter, there are many more animals to look at, and often wind and waves to make the paddling more fun, too! Just be sure to check the weather forecast for the expected wind, so you don't get more fun than you like. And read this.