Bit the bullet and bought a 15d/3GB plan to test. Not cheap - even after discount - landed on my 28° MC at A$81

. But that’s still cheaper than ship wifi…
Closing the loop on GigSky Cellular at Sea (CaS) option eSIM.
TL;DR:
Not cheap, can be clunky to use but a good option to provide some connectivity when not paying for overpriced ships wifi and you’re spending several days at sea away from land based mobile reception.
So getting a Cruise+Shore bundle (for compatible ships) is something to consider if you’re not forking out for wifi. On our recent cruise, it wasn’t possible to pre-purchase wifi and onboard and it was US$25 pd one inboard (not happening on a 12 night cruise).
We already had plenty of eSIM data to cover time ashore and when within range of shore based mobile, so just needed a little data to keep on top of emails etc.
The challenge with CaS is there’s a lot of moving parts involved - mainly the ships infrastructure. Literally at the top will be satellite connectivity into the CaS network. That is a totally different satellite connection for wifi. Then throughout the ship, there are mobile phone repeaters (think mini cell towers) that your phone connects / roams to when moving about the ship. In our experience, the CaS system didn’t work properly in our cabin. It connected, but couldn’t get data. But most common areas seem to work ok.
The real hassle was needing some manual intervention to the still get it to work. The phone might show it was connected at 4G/LTE but no data. I figured out that forcing the data to 3G worked…

. Whether that’s ship specific configuration issue or a common problem - I don’t know… Anyway, it served its purpose.
Seperate to GigSky, I did discover that I could receive SMS for free on my home amaysim line over CaS, so that was very handy to know.