More on ship's wifi vs. local port carrier signal
If you are using the ship's wifi while out of range of land based carrier signals, you may need to turn off your wifi reception when you get into port or close to land to receive the local carrier signal--assuming, for example, that you are willing to pay, for example, Verizon's $10/day travel pass. If you don't turn off wifi, the phone may default to the ship's strong wifi signal, and if you are on a ship that charges for wifi and you haven't signed onto the wifi, you won't get anything. So this is a case of the ship's wifi "blocking" your port's cellular service, but it is blocking that is easily defeated and the reason for this blocking may be that the phone is programmed to default to wifi because in most cases wifi is cheaper.
Also, Seabourn insists that Cellular at Sea is entirely an offering of the cell phone company and not of the ship. I am a Verizon customer and I found Cellular at Sea offered when out of reach of normal phone service on a Canadian maritime cruise but not a Med cruise.