I understand that, but at sea the SIM service is provided by the cruise ship communications, which is the same as they use for WiFi. It’s all satellite. Speeds on your SIM data will be comparable if not slower than the WiFi provided by the ship. As mentioned, most cruises throw in free Internet as part of their incentives.
Discussion
Nope, you’re not understanding it.
If you’re happy with what you’re doing ignore this, if you’re curious, research more, it’s much cleverer than you think it is.
Help me understand. How are you using eSIM on a ship without cell towers in the ocean and not using the ship’s infrastructure?