On the vessel: identify what carries the connection.
Shipboard Wi-Fi is a way to reach the vessel's network; its internet connection may use a separate satellite or shore-based service. Onboard systems can combine those networks. Access, allowances and availability depend on the arrangement on that vessel.
A travel eSIM for cellular data ashore is a separate product. Buying one does not buy access to the vessel's Wi-Fi or provide mid-ocean cellular coverage. Ask about onboard access before joining.
Source: Inmarsat: onboard connectivity using satellite and terrestrial networks.
At anchorage: plan for a gap.
Seeing land does not confirm a usable connection. Check the selected plan's supported network and coverage, but do not make an essential handover depend on a signal at the anchorage or during the launch transfer.
Before leaving a working connection, save the next meeting point and current instructions. Agree when the coordinator should expect the next update and what to do if it does not arrive.
In port: country coverage is only the first check.
A country on the plan's list does not guarantee reception at a particular berth, inside a terminal or throughout the onward journey. For example, the FCC explains that its U.S. mobile coverage maps describe outdoor or in-vehicle service, not indoor coverage.
Check the connection where you are permitted to use the phone, then send the arrival update before starting the next leg. Keep the local contact information offline even after the phone connects.
Source: FCC: what the National Broadband Map shows.
At the airport: treat each country as a new check.
After a flight, confirm the intended data line and plan coverage before relying on messaging or navigation. Airport Wi-Fi may be another option, but have the onward flight and pickup details saved before you need it.
For a connecting flight in a different country, repeat the check. Coverage at the departure and final airports does not establish coverage at the connection.
At home: check which line is active.
Confirm that the home line and your preferred data line are active as intended. Review any travel line still enabled and keep the order details until any support issue is resolved. Follow the device and provider instructions when deciding whether to retain an eSIM for another journey.
Agree the next update before the connection changes.
Write down who expects to hear from you, by which method and at what stage. That makes a temporary loss of mobile data easier to handle during a crew change.