I don't consider your cottonmouth description to be aggressive behavior. We are giants to most snakes - something 100x bigger than a cottonmouth or rattlesnake. They don't want to get stepped on and crushed, so they're trying to get our attention. 'Hey, I'm here, don't tread on me!'
There's been a study or two with prosthestics where researchers tried to elicit bites.
In this study, only 6% of rattlesnakes bit when stepped on with a fake leg/boot. Most tried to flee. As mentioned by GladMax, the gross majority of venomous snake bites
in the US (and I'll venture Canada) happen to people deliberately interacting with snakes (trying to move them, capture them, kill them, photo graph them, etc).
In two years of working on wildlife research in the Everglades, mostly studying and trying to control pythons, tromping through backcountry for miles and miles, wading through swamp and marsh up to chest deep, I encountered all of two cottonmouths that acted as you describe, which I'd call being visibly defensive as opposed to relying on hiding or leaving. I always backed off, and we were all fine. I encountered many large pythons (8ft - 15ft), only one of which ever bit someone before we grabbed it. The ones that lunged and struck were all being grabbed - their first instinct is to hide, and their second instinct is to flee (which they do extremely fast). Even snakes like cottonmouths, watersnakes, racers and ratsnakes that sometimes rear up and lunge, which I categorize as being defensive. When you back off, they generally stop and try to leave. I had similar experience during a summer in Okefenokee.
When I worked in the Ozarks there were tons of copperheads, and timber rattlers were common enough. We practiced snake-safe bushcraft - always look where you put your hands and feet. These snakes tend to hang out along and under logs because mice travel along logs, so I always look on the far side of a log before stepping over. When our botany crews encountered a venomous snake in the plot they were surveying, they just assigned one person to watch the snake and make sure everyone kept their distance. The snakes generally went on their way with minimal fuss.
Snakes dropping into boats is real, but it's because their natural escape behavior from sunning is to dive into the water, just like turtles. They lack of comprehension of canoes is certainly unfortunate, and I sure wouldn't want a venomous species feeling trapped at my feet in the boat. And, I have also witnessed this behavior in which swimming snakes approach boats sometimes - it's honestly a head-scratcher but I try to keep my cool and give them space and we go our separate ways.