i second tugsailor, but i use ATT intl. plan
AT&T has the best service and widest coverage, but it’s the most expensive.
Why is their phone not in airplane mode already? Searching for signal is a MASSIVE battery drain so they should always be on airplane mode while at sea and is they don’t want roaming charges in a foreign port then just stay in airplane mode. There’s nothing to forget during port operations because airplane mode is already on.
It’s also possible to disable international roaming, but just leaving the phone in airplane mode is the best option.