Is it safe to activate a cell phone while you are inside of a pool of water?

As long as no water gets on the phone, then yes. If you want to use it underwater and stuff, then you can purchase plastic cover things from phone shops.

Cell phones run on battery power - but keep in mind that they are little tiny computers that do not like to get wet. You will notice a little white dot near or under your battery. This little white dot turns a different color when the phone is exposed to mositure it will void the warranty unless you have insurance.