Like kerf, I really don't have an answer, but the owners put a truckload of gravel down in front of my shop on top of plastic sheet with the idea it would prevent weeds from growing.
When my bike was only HT powered if was a great effort just pushing the bike to the road and left deep grooves in the gravel. After adding a 650 watt electric front wheel to make it a hybrid I can walk it to the road without effort, then start the HT.
After some practice I found I can ride through the shop door full throttle on the HT and electric, but stability is a problem to the point of being scary.
In desert sand, say 2-3" deep the HT handles it nicely. I use dry while spray lube on the (#41) chain so it does not collect sand that I can tell.
Anyhow, on a beach I suspect control would be as much an issue as it is in deep, loose gravel except where the sand is damp.