I keep my sand/water table filled at all times. It is available to the kids during free play in small groups. I change out the stuff inside to match our current theme or topic.
I keep it inside a plastic blow up pool that is square shaped. I have never had an issue with it being messier than the pool can contain.
I LOVE sensory bins/tables!!
Can you please post a picture of yours!
For me I a few times in the winter will set up my water table in the kitchen on the floor but I use tons of blankets. This activity is only for an hour or so!