If we only stay for a day, can we still go to the pool?

Yes it's a nice pool. Try the Italian restaurant next door. It's excellent

As long as you rent a room you have use of the pool just like any hotel. Pretty standard. Not specific to this hotel.... but if ever going to a hotel just for the pool call ahead and ask if it is open and if they have just a day rate charge.... just for the pool with no room.

Sure