Why would a snake follow a kayak?
likely they are trying to sun themselves especially in cold weather. The water is cold and they immediately try to get back on the boat.  becasue they are not as afraid of you as other snakes they are harder to discourage. They also probably can't tell the difference between your boat/dock and a log.

as for following you I can see three possible answers. 
D's answer about eddies might be it. 

Or they may be doing the same thing birds do with tractors, your kayak and paddles stir up the water and they are after the animals your movement scares up. A lot of small predators have learned following human machines can be a good way to find an easy meal. 

Or they might hope you are a crocodile that will scare up small animals or leave scraps they can eat. Answering Hans will help understand what is happening.