DVC members can hop to any pool except SAB or AKL, except during blackout periods (Thanksgiving, over Christmas, etc.) when no hopping is allowed. You are supposed to call the resort first and check (we didn't, we were in the MK) and make sure the pool has ample room and you may be asked to leave anyway if the pool is crowded. Reports on calling ahead seem to get more from desk "huh?" than any good response. You may be asked to show your room key and/or blue DVC member card. Its a perk that can be suspended at any time.
The Poly Volcano pool was nice, but my kids were less impressed than we imagined. However, we checked out that morning, so we no longer had our BW room and were spending the day at the MK and eating at the California Grill that night. It was actually kind of a hassle - carrying swimsuits, changing in the bathrooms, having all our stuff with us (my husband and I didn't do much swimming, we were watching stuff), changing back in the bathrooms and then having wet swimsuits to haul around. I can't imagine us making it a regular practice.