Replying to Royal Caribbean/Drinking

So i'm going on a mediterranean cruise in a few days, and I wanna be able to drink. Problem is that its 21 on the boat, unless your rents sign a waiver, then its 18. They won't and I'm 17.

So is it that hard to get served on the boat?

My plan was to buy some in france or italy, and either drink it then or bring it on in a water bottle or camelbak

I don't think it will be hard to buy on shore, biggest issue will prbly be my rents, cuz they said they will let me drink, just not unlimited.

So any ideas or anyone with experience that can give advice?