Spiders
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.
I hate to advocate killing something just for being what it is, and it's not like the poor creature deliberately attacked you, it fell in the boat by accident. But what else can you do? Have the heeby-jeebies for the rest of the day? It will become fish food, all part of nature's great cycle. Orbweavers are pretty big, as spiders go, and really neat if you take a close look at them. Or maybe just settle for photographs.
-- Wikipedia
This is a listing of some of the tiny critters that you might find. I'm not including all the invisible biting nasties that you find around the water. That's what bug spray is for. Above is the horrible Greenhead fly - the bane of New Jersey summers. When one of these bites you, you know it.
More: Bugs ...