I've surfed behind my 211 for four summers now. The boat is on the water for three months solid and used nearly every day, mostly to surf. If you are only taking on water when you surf, there are three common places where water gets in when the boat is really keeled over. 1) As you have described, the side vents. 2) The black bumper rail especially along the stern. 3) The rear bilge pump out flow.
Solve this by 1) partially occluding the vents on both sides by removing the plate and covering the bottom 2/3 to 3/4 of the vent with a thin rubber gasket material and replacing the plate. This leaves enough space for air movement and the blower to function and prevents water from getting in. 2) With a silicone based sealent run a bead along the under side edge of the bumper rail all along the stern (especially around the metal decal in the back) and up the sides of the boat for about 6 ft. 3) You can either get a new bilge pump with a stiffer impeller, place a one way valve in the hose, or, if you have a good memory, you can fasion a small rubber or plastic plug that you put into the exit hole on the starboard side while you surf goofy...just remember to turn off the pump when the plug is in and remember to take it out and turn the pump back on when your done.