I have woken up with BPPV just from sleeping in the wrong postion. After a google search I found some simple movement exercise positions that worked amazingly well at curing it. Big credit to the internet for providing very helpful information.
As for heavy crashes from a foilboard, mostly I get whiplash. But as I get more experienced it happens a lot less often. An impact vest really helps and a helmet perhaps too.
One thing to try is: when you realize you have lost control on you foilboard and are going to crash hard, sheet your bar in which will rip you upward (without your board) into a slower high jump. You may end up a lot farther from your board, but you wont be hurt as much.