This subject makes me question some of my own experiences with cottonmouths.  It also brings up some questions.
I know most wild animals (reptiles included) can be territorial to certain extents. How far, if any, would a cottonmouth go to enforce its' territory? And would a female with young near by be more aggressive?
I have had a cottonmouth swim what seemed to be well out of its way to investigate me while wading around at the river. It did go into an aggressive/defensive mode when it got close. I am on the fence about its intentions. Was it being territorial, curious or was it just coincidental that it swam right to me? Either way, it was enough of a display to get me to steer clear of it and move on my way.

Think about it this way- while snakes are decent swimmers, their metabolism and muscles aren't suited for the constant elevated effort that swimming usually requires.  They normally slip into anaerobic metabolism when making that kind of effort, build up substantial lactic acid in their muscle tissue, and then rest periodically to breathe off the 'burn'.

I've had many snakes swim up to my person or boat while fishing, rest against me or the hull (I just watch them) for a minute, and then continue swimming on (though I've NEVER done that with a cottonmouth, lol).

I think that 99% of these cases are instances when a snake is tired of swimming, sees something big enough to rest against for a minute, and swims towards it.

Van

Cottonmouths quite oftencome out into the water to check for potential prey. Weather I'm in a boat or in the water, the vibrations alarm the snake and it heads toward the source to see if potential prey caused them. After checking me out the snakes usually either turn back or cross the water in front of me or behind me but they don't come up to bite me.

I actually read this in a book today: A study was done out on the Savannah river (I believe) in east Georgia to test the aggresiveness of cottonmouths. 50 snakes were tested in the experiment. First the scientists stepped on the snakes with snake proof boots and only 5% of the snakes bit the boot that stepped on it. The rest of the snakes displayed their cotton mouths, vibrated their tail, and excreted a foul musk to scare the scientists away. After being stepped on, the scientists used a fake arm to pick up the already ticked off snakes and only 30% of them turned around and bit the arm. The others tried to slither away and ecape the grasp of the fake hand. Apparently the snakes won't waste venom if they absolutely don't have to and don't seem to be as aggresive as everybody says.

I stepped on a Cottonmouth 0ct. 25th a few years back......
I was leaving the River and being that late in the year,I had let my guard down as far as keeping an eye out..............was in a really bad mood from something that happened at the River.
Just was not thinking anything,and started walking up some Limestone bedrock that was exposed...........
UH-OH rubber-hose feel under my feet.........................I seen it was a Cottonmouth,I screamed-it just came out-lol.........jumped straight up off it
the Snake swung its head back toward my leg,with its mouth open but did not bite me......I had stepped right in the middle of a full grown Cottonmouth........
My Dad and Granddad years ago had a Cottonmouth try to get in their boat,in the middle of the Lake....
so I guess it had some how got out really deep and got tired and wanted out of the water....