I just finished a cruise on Celebrity. AT&T cell service is available (for a fee, of course) on some of the Celebrity ships, but not all of them. Our cruise package included unlimited internet which worked well. No ethernet port in the cabin, just wireless access. There was a 1 device limit, but you could switch between multiple devices (i.e.: only 1 device could access the internet at a time). Just did email and Facebook and stuff like that. Did not try anything like a remote session.
But, I could not establish a connection to a VPN server using Private Internet Access. Not sure if this is deliberate or not. Obviously, YMMV depending on which cruise line / ship you're on.
Harry Z