The vendors accept cash. There may be some who accept credit cards but not many.
For the drinks you purchase it will be charged to your sail pass.
As for what to do with your belongings- we go with a group so someone is always by our stuff but if you are going in the water & had no one to watch your stuff you can purchase plastic waterproof cases that you can put around your neck. I know RCCL sells them in one of there stores on board. They don't hold a lot but you can fit your sail pass & money in there. Hope this helps!