I'm cruising soon, and I spoke with a real person at AT&T today instead of relying on the limited information on the website. The AT&T person told me to turn off cellular data when I wasn't in the US (including aboard ship) so my apps that run in the background will only use WiFi for data. I chose not to purchase the AT&T plans, but rather to purchase the social media Internet plan on my cruise ship. It's less money and it has Facebook messenger, which is all I need to communicate with my friends and family no matter where they are. I've done that pre-pandemic several times. While the speed is abysmal and posting a photo on Facebook will not happen for hours (if ever) while at sea, the messenger option works great and it provides me peace of mind knowing I'm not going to have any surprises on my cell phone bill.
I always have my roaming off 100% of the time to avoid any surprise charges (especially helpful with rural areas).
I would recommend visit https://www.att.com/support/article/wireless/KM1001002/ (a help page about Talk, text, and use data on a cruise). There are telephone numbers there to reach out directly with AT&T.