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.
-- Wikipedia
Common Dolphin
On the other hand, the Common Dolphin prefers warmer weather, and can be seen all through kayaking season. They go south for the winter.
-- Wikipedia
These are some mammals you are likely to see in and around the water.
More: Mammals ...
Orbweavers
If you are trying to escape the sun in the shade of overhanging tree branches, you are inevitably going to get spiders in the boat. These are almost entirely harmless orbweavers - they are not aggressive and don't bite, and are not poisonous to humans, at least no more poisonous than a honey bee. That said, our brains are programmed at the most basic level to fear spiders, and I don't like them any more than anyone else. Just calmly knock it out of the boat.
More: Spiders ...