Your battery need depends on how you plan to use it. If you trolling back and forth for 4 hours at a stretch, the larger battery makes sense. But if you are just transporting to your fishing site, the smaller battery is more than enough. Your Watersnake runs at 9 and 20 amps. You'll be surprised that the 9 amp setting will move you along almost as fast as the 20 amp setting -- probably only about 1 MPH slower (until you adapt your controller).
Here are some relevant speed comparisons vs power consumption with the Hobie Evolve, which I believe uses an 11ah battery. These figures are for an Outback, but notice that using only 25% of max power moves the boat along at almost 4 MPH. Your PA 12 should be within a couple of tenths if the Watersnake is comparably efficient.
Watts...................speed MPH
--26----------------2.3
--32----------------3.2
--75----------------3.9
-101----------------4.3
-160----------------4.9
-210----------------5.1
-325----------------5.5
If you keep your speed moderate (3.5 to 3.8 MPH), your 20ah battery should give you at least 2 hr continuous running time IMO. It really doesn't take much power to push a kayak!
