smashed thumb ... how soon back in the pool?

First and foremost:  Is there an open wound?
Two parts to this, the first being introducing any of *your* blood into the pool. 
Second being the risk of infection for bathing your wound in the unknown pool environment. 

After that, it's a question of comfort and whether swimming puts pressure on that very angry part of your anatomy. 

I always seems to happen that way, right?  I agree on staying out of the water a few days, then maybe you could Tagederm the thing once it's a little better but not still fully healed.