In the water I've dug holes deep enough to be knee deep in the hole chasing an elusive target. That puts me a lot more than knee deep in the water. A little kid who may come right behind me could suddenly find themselves in over their head. What if they can't swim or just panic? I've seen several times where someone riding a bicycle came across beach holes filled but not packed in and have their tire sink into the sand and get pitched from the bike.
100%
I was recently digging "an elusive target" on a hard packed clay lake bottom and was already chest to neck deep chasing this signal. I had to lean off to one side or go way over my head (crater was on a deep slope) and I turned to dig from another angle, lost my balance a little, and sort of got stuck for a few seconds of panic there. I managed to somewhat cover the hole because you know those craters won't fill in that easily in fresh water. If a kid or anyone for that matter was walking near and stepped into it they could possibly drown. Oh and by the way, it was just a clad quarter (lol)
So yeah, I always fill them in because if someone got hurt or worse I'd really feel s@#ty about that! HH