Not sure how fast you are talking about going... but I had the feeling the orignial post was for lake use... if for racing it's different a different story to meet the rules.
In my mind, motorcycle helmets are very poorly designed for water sports, and I've tested a few... haha. They are made to bounce down pavement... not water.
For lake use, I'd be tempted to look more at good bicycle type helmets (NOT Canadian tire type stuff). They are not DOT/Snell approved, but made similar (in many cases cost much more). They have many more and much larger vents in them so they are not race legal... but this lets in much more air flow so you are not as hot, and in the event of a crash they are MUCH lighter to help with whiplash type injuries and less likely to bucket I'd think because of the big vents.
Here is one to give you the idea.. could remove the visor if you're going fast or have no windshield:
carbon fiber, lots of vents, big holes.
To add to one of the comments I saw... from person experience, bucketing becomes an issue at about 65 mph. If you've got a helmet on above that, you should certainly have a lifejacket that has a water deflecting colar. I don't know much about helmet restraints.