Cell phone/internet usage on a cruise

put your phone into airplane mode then turn wifi on and you should be ok

So turn on the international plan AND turn the phone on airplane mode and that's it??  

The international day pass plan does not cover cruise ships. If you need cellular service on the cruise ship, consider getting a cruise ship plan but roaming on a cruise ship is expensive. As suggested, it is best to put your phone in airplane mode when on the cruise ship. If you have the ship’s WiFi service and have WiFi calling activated on your phone, you will be able to call and text over WiFi with no roaming charges. I just returned from a cruise to Iceland and WiFi calling worked great.

To be clear, ATT's IDP can or cannot be used while a ship is at a port in a country covered by ATT's international day use play ($10 per day?)

First, you don’t need any kind  of plan to connect to WiFi, regardless of whether you’re on a ship or not.  WiFi is WiFi, just turn your cellular off.  Regarding cellular, it depends on what carrier you’re connected to.  If you’re connected to a carrier in the country you’re in, then IDP would apply.  But if you’re connected to the cell service on the ship, then it likely wouldn’t apply, because it’s the ships carrier, and the ship could be anywhere.

But if you’re connected to the cell service on the ship, then it likely wouldn’t apply

The IDP definitely does not cover the cruise ship’s cellular service. Cruise ships are supposed to turn their cellular service off at ports, so connecting to the ship’s cellular when in port shouldn’t happen but bottom line, the IDP does cover port stops provided you connect to a land based carrier, not the ship.

Thanks. So I should be able to identify the ship's cell carrier to avoid accidentally connecting to it?

If there a good app you would recommend that locates free wifi hot spots at various ports around the world?