Hey y’all, my pool is still under warranty and with that warranty the pool company requires us to have them professionally close the pool, so, the pool company is closing our pool next week. They offered us two types of closings, underwater and dry. We don’t want to drain our pool so we are paying an extra $50 for the underwater closing. Can anyone explain the difference and why the pool company uses these blue, 2-liter bottle looking things with threads on them?