When learning, I used a helmet and a full suit, very fast, eventhough starting in a summer shorty.
As I would otherwise cut my feet and legs on the carbon foils, and I had no idea where the board "went" when falling (as I was alone here, learning).
Also, it was stinging jellyfish season, so I was almost hurt to tears every time (could not sleep, and my body was steaming moist from the arms and legs all night) - so used a full suit for a while.
Now, I dont wear a helmet (but might do it as the board WILL continue on its own and hits you now and then), and as I ride "more" than when starting, I will also be able to foil in shorts or shorty.
And you dont get "cut" by the foil anymore - that was just in the beginning when learning and you were fumbling stupidly around sometimes...
I am lucky, to have quite "closed" ear channels (dont know the correct word), so can easily get wax that can close - but at least never perforated eardrums - so dont need anything to protect
Having said that - I can always recommend a helmet though, as the board WILL hit you quite often.
Also, falling (MANY many times) at high speeds from almost 1 meter higher than on other boards, will make your neck really really sore for days.... Even WHEN you are used to it
You get such a power crash slam bang when you land - and when it is winter cold water like right now (I never use a hood), you (well, I) dont "relax" when hitting the water ha haa
Not sure what good an impact west will do though ?
But a helmet, indeed