Navel piercings take 6 months to a year, sometimes longer, to heal. You should wait until it heals to change jewelry and to swim in a pool. If you change the jewelry too soon it can get irritated and prolong healing time. Same with if you go swimming, You can get bacteria into the piercing which gives you a chance for infection.
If you really want to swim while the piercing is healing, they make waterproof bandages that work great. My sister went swimming in a lake while hers was healing, she just put tons of waterproof bandaids over the piercing and water never touched it.
25 piercings, 2 of which are healed navels.