Anytime when skiing.
Been out today in some big waves, with my lid on.
And one of the lads was found face down in the water.
Apparently he was wave jumping on his mates ski, and cracked his jaw off the handlebars.
Some of us jumped off our ski's and turned him over and got him to shore.
He was out for the count but still breathing.
He swallowed some water but he will be ok.
So helmets are as important as life vests, imo.