Swimming is not one of the actions provided, so you would have to make your own..
Create a swimming animation
Decide what you want the swimming to look like
make actions based on that..
For example you could switch to 8 direction behaviour when overlapping the water..
just add the animations and you are done..
But you'd probably want more control over the swimming part, which would consist of adding more canditions and attached actions..
You might want to try searching the forum for earlier examples..