Taking time to get out my phone to take a picture of a snake in my kayak. Not going to happen. Taking .002 seconds to fling snake a quarter mile with my paddle is more likely.
I've seen ones in the swamp that were jet black on their top to their sides. That body shape and head looks like one.
If it's black snake, swimming the water and gets into your boat. It's a Cottonmouth. Having them fall out of a tree into the boat is also a good indication. Of course could be wrong, happens. Would like to know what black snake commonly swims and are aggressive not a Cottonmouth. (not afraid of people)