Ok let me help you on a few things on snakes. First by correcting some falsehoods above.
True--Cottonmouths are territorial which means they stand their ground. They warn you by tightening up and opening their mouth wide and showing their bright white inner mouth that stands out brilliantly against their darker outter body. Many snakes run away and so to will the cottonmouth at times. On land mostly they will try to give you this fair warning because they arent fast on land. In water they cant do this so they typically panic and run when threatened in water. Sometimes they will run for the nearest dry object which means your canoe. This doesnt mean they are attacking you or the canoe but it often means this animal with a pea size brain is just trying to escape. That being said having one in a canoe with you isnt safe either.
False--Cottonmouths swarm or are found together. Well wouldnt that make the territorial fact not work? They arent just territorial towards humans duh. They dont much like other snakes and competition either unless they view them as dinner. So chances are if you are seeing a lot of snakes in a small area you are seeing something other than cotton mouths.
False-- 6 to 8 feet long cotton mouths. The longest ever recorded was barely over 6 feet. This is a rarity in cotton mouths. They rarely ever see 4 feet because they are a wide growing snake instead of a long growing one. A 5 footer would be as wide as a 12 foot python where as a non poisonous banded water snake that is 5 foot would be only slightly wider than a common garden hose.
False--Copperheads in a river or pond. Not sure who mentioned that but this is very rare. Copperheads like rock, and holes, and logs and forest areas. They will venture to a creek during a dry summer to find water if there is no standing pools to drink from after a raid but copperheads ARE NOT water snakes in anyway shape or form. This is what people call banded water snakes when they see a water snake because they are afraid of all snakes, therefore all water snakes must be poisonous. False.
Here is some easily found info you can back up by googling it. Many water snakes including the brown water snake, the banded water snake all varieties, and several other water snakes LOOK a lot like cotton mouths. Their colors can be at times identical, their patterns very very similar and their habits "in water" identical. So how does one tell the difference safely?
1)The cottonmouth is a pit viper. A pit viper has poison sacs in what you would call his cheeks. This gives his head a spade shape. Non poisonous snakes do not have this therefore dont appear to have cheeks.
2)The point of the pit vipers nose does not slope downward. It comes out to a point even to its eyes whereas a non poisonous snakes nose will be rounded downward and come to a point lower than his eyes. Of course you got to get a little to close to tell this feature.
3)Cotton mouths grow wider than they do long. If you see what appears to be a short yet wide snake in or around the water that might just be the one to leave alone. If its 4 foot long or longer and looks like a cotton mouth but is as big around as a garden hose its a non poisonous water snake. The only skinny part of an adult cottonmouth is the last couple inches of his tail. A really big cottonmouth's tail is comical to look at as seen here.
http://www.itsnature.org/favicon.ico
4)Colors and Patterns--this is where the problems exist. There are many types of water snakes that have similar color shades and similar patterns to cotton mouths. This is why people use the term "Water moccasin" to describe water snakes because they have no idea what exact snake they see swimming actually is. When someone screams "omg water moccasin" what they are saying is "oh no im terrified of snakes and they are all poisonous so im going to panic for no good reason".
Here are some pictures of Water snakes poisonous and non poisonous. At first glance they all can be mistaken as "Water Moccasins" lol. But if you use the above info, not just descriptions, but how does the snake act and live. You can go out there, take some pictures, and make an educated non fear induced decision on weather to swim or not.
Cottonmouth.....
http://t0.gstatic.com/images?q=tbn:ANd9GcRZ3KJe-VMK_tpBQgYOshKAs9Z0x5NGigqYTZahm2ioitvmAJpn
Banded water snake.....
http://t3.gstatic.com/images?q=tbn:ANd9GcSPAUTx-B6Y3lavuVbtBOZFfc3ciD-mWchoInU7WeAqYS0g-YG_5w
Brown Water snake.....
http://t2.gstatic.com/images?q=tbn:ANd9GcSuPckGkb4ttBpApkp-s8fFCF8mjnNjxzQXBS0PDsKB9PgoxLh4
As you can see they sure do look similar and if they were in water its even tougher to tell. But the fact you saw lots of snakes in the same area tells me you have more social species and not cotton mouths. Doesnt mean there arent some there, always be wary. But its highly unlikely there are lots of them, and even more unlikely they are going to do anything to you but run when they encounter you.