I have had a selection of helmets on my boat since 2005 and had lots of feedback from a couple of dozen people. Lots of sizes and shapes with lots of crashes. A couple of concussions and several more regrets. Let me take you straight to the answer about helmets and watersports:

Yes, they offer protection from impact (for skull and ears) that far outweighs any risk of catching an edge. However, they MUST fit properly. If they don't, any play will result in the water slamming the helmet into the head.

I have not had helmets for most kids simply because such sizes were virtually impossible to find until recently. I found one kid-sized helmet, but it turned out to be a poor design (too wide) and we never bring it anymore. My teenage nephew has usually worn a helmet, but lately has been skipping it because he says it blocks him from spotting a landing with some new trick. This isn't around his friends, so I'll believe him.

Most of my helmets have been Pro-Tec Wake helmets. However, last year I got a Bern "hard hat" for a new rider and it's much better. The Pro-Tec have comparatively thin padding with an internal strap for adjustment. The strap's fasteners are very flimsy and slip away far too easily. The Bern has no internal strap, replacing it with two layers of thicker internal foam, graduated in density. This is a MUCH better solution for the full range of impacts from soft to hard.

The Pro-Tec helmets have better venting on the ear cups than the Bern ones. I simply drilled out slightly bigger holes in the Bern ear flaps.