In the surf I always wear a Gath Visor Model helmet which I have had for over 15 years, as a "hit to the head" is usually obligatory in the surf. I was very grateful I had one on the last time I went out.

On flat water I usually don't wear a helmet but if it is looking a bit O.T.T. then I wear it.
Managed to hit myself fair across the back of the head with the paddle while SUP surfing last week when I fell of and the wave got me. A lucky escape as I had the helmet on.