I usually use these Polar Insulated Water Bottles for cycling, but they also work very well for kayaking. You can wrap the water proof Velcro around them so they stick to the bottom of the cockpit. The insulation isn't really a factor because if you are out on the water for several hours, any cold or hot liquid you put in it, will be at ambient temperature eventually.
The good aspect about this bottle is that they have a wide mouth, so they are easy to pee in, if conditions are not too choppy. It is useful to have multiple colored bottles, so you don't confuse which one you peed in...
One thing that is not so great about this bottle is the drinking cap which can get loose and sometimes will let the salt water mix in with your drinking water. You then better hope there is a place you can get refilled.
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