These are some mammals you are likely to see in and around the water.
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Muskrat
Muskrats are common across the state. While considerably bigger than plain old rats, they are nowhere near the size of beavers, which can get to 50 pounds. Muskrats have tails that are slightly flattened from side to side, unlike beavers which have broad flat tails. Both animals build conical houses in the water, but only beavers build dams.
-- Wikipedia
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River Otter
Otters are aquatic weasels. You may find them climbing around on banks, or splashing around in the water, especially around dusk.
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Harbor Seal
Harbor seals are not uncommon in the winter, but kayaking is, so don't expect to encounter them when you are out for a paddle.
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Osprey
Ospreys, or "Fish Hawks", are surprisingly common, and can be found just about anywhere. You'll typically see them flying high over the water from a tree on one side to a tree on the other, occasionally diving down on some unlucky fish. Ospreys are usually seen alone, but mating pairs stay together, and I think young siblings stay together for a while as well. Ospreys are one of the biggest birds that you are likely to see. A single species has an almost worldwide distribution - pretty much everywhere except Australia.
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