Can anyone think of ways or excuses to hide my belly piercing by the pool from my parents?

Oh that's a tough one.. Depends on how long you've have it, if you've had it for a while, you might be able to take it out for a little while.. but everyone is different and you might close up, so Im not sure if that's great advise..

Hmmm I would say wear a one piece, but when you get wet the suit clings and it's totally obvious.. An excuse you can make up for a reason why you don't want to swim.. say that you have your period and it's flowing really heavy and you have cramps and you don't want to swim.. if you already had your period and your family is in tune to when you already had it (creepy lol) say you don't feel good and just decline swimming and act sick the whole day

I got my belly button pierced when I was 19 and hid it from my parents too (I was still living with them while going to junior college).. I had it for a week when I accidentally lifted my shirt up to wipe my hand, and my mom got a glimpse of the metal, and totally freaked, and so did my dad, they fed me crap like.. "you're living under our roof and you should live by our rules!" it was horrible, I took it out the next day, wah I only had it a week, it was sooo hard to take out, it was so cute!

You could wear a clear retainer instead of metal jewelry, but it's really not recommended in a new piercing.
If you do decide to put the retainer in, then it would be very hard to notice it.
Another option is wearing a one piece- although you will probably be able to see it some through the material.

Wear a one piece suit or be honest with parents,

Just be honest with your parents. If it gets infected or injured, you are going to have to tell them anyway so they can take you to the doctor.