There is a particular kind of disappointment reserved for the third day of a cruise, when you finally give up refreshing your messages and accept the ship hasn't been connected to anything since Tuesday. This one comes with an honest warning attached, because the marketing around "connectivity at sea" isn't always straight with you.
The two kinds of "at sea"
In port, your ship is basically a very large building on land. Your eSIM works exactly as it does anywhere else - it finds the local shore network and connects like normal. Out at sea, between ports, there usually is no shore network to find. Mobile networks are built for land; the water in between is served, if at all, by satellite, and your travel eSIM doesn't include satellite coverage. No eSIM sold for ordinary travel does.
Why the ship sells its own wifi
That satellite gap is exactly why cruise ships sell their own onboard wifi, usually at a price that makes airport roaming look generous by comparison. It isn't a rival to your eSIM; it's solving a different problem. Your eSIM is a phone-network subscription; the ship's wifi is a satellite internet service billed by the ship, not by us. If you want to be reachable mid-ocean, that's the only route, and it's worth deciding in advance whether you actually need it, rather than buying it in a panic on day one.
Most cruise itineraries spend more nights in port than at sea, which is worth knowing before you write off the whole trip as offline. A seven-night cruise with five port stops and two sea days means five ordinary land-network days and two genuinely quiet ones - closer to a normal multi-stop holiday than an off-grid expedition.