Wide open sandy beach, 25 km fetch. Water is probably freezing cold and no idea if the summer lake level is the same as the winter pic, potentially higher water would mean the beach would be gone. You can see all the logs up at the high water line, but that's probably during Spring.
No idea how much wind Williston Lake gets but I'm sure it gets some.
1.5 hr drive, but the Williston Lake Resort just up the road has glowing reviews as well as an RV park.
EDIT: looking at the water levels they bottom out in Spring and rapidly rise as Summer hits, total swing of about 11m. So by the time you'd dare enter the lake the water level will probably be too high for that spot. https://wateroffice.ec.gc.ca/report/his ... mode=Graph