I use a multisport helmet for whitewater kayaking.
This one...
http://www.sweetprotection.com/helmets_trooper_hc.htm The ear flaps come off very easy, but for wakeboarding they would be needed I would think.
Very cool helmets...the best part is that the liner simply pulls out and you can wash it in your washing machine.
That is a HUGE plus....lake water can get funky really quick soaked into a helmet liner.
I have a few grateful head helmets, and a few other brands. Good stuff....but too much area for a high speed impact.
I also have the Sweet Strutter for playboating...but the bill would be sketchy in a high speed crash.
http://www.sweetprotection.com/helmets_strutter.htm Here are a few pictures of me being stupid behind my boat in a kayak...believe me at any speed at all that bill is a mistake. I about ripped my head off.
For now on I will use the helmet noted above.
http://trask.smugmug.com/gallery/676218/1/29263044