Roamless eSIM relies on getting cellular signal from land-based cell towers to provide data connectivity.
On cruises, when you are open waters, it is unlikely for your Roamless eSIM to consistently get cell signal to keep you connected.
When you make port (as long as the country/region you are visiting is among the 200+ Roamless destinations), your Roamless eSIM should work without issues and provide reliable data connectivity like usual.
If you are connected to the ship's WiFi, you can also use your Roamless app to make phone calls to 200+ destinations from inside the app (as long as you you have pay-as-you-go credits available in your account and the ship's WiFi offers stable connectivity).