If you use your phone on the ship, connected to the ship's cell tower, the charges bill through AT&T. If you have a plan, the billing will be through the plan. Be very very careful since it would be easy to blow through the included data. If you connect to the ship's cell tower and don't have a plan, you are billed standard AT&T pay-per-use rates ($3/minute talk, $0.50 per text message, $1.30 per video or picture message, $2.05/MB data).
Alternative: If you just connect to the ship's wifi/internet, you can use your phone for wifi calling. That has to be set up prior to boarding the ship. And you need to have a wifi plan on the ship.
We have used cruise ship plans for voice and it worked very well (better than wifi calling).