As Tdenson says Wifi on Cruise Ships is unbelievably expensive about 50p per minute though you can buy it in bulk and get it down to about 35p per minute if you buy it at a 100 minutes at a time

And because it's via Satellite it's pretty slow

You are joking. The concept that the people sitting behind the customer service desk on a cruise ship would have the slightest clue about such things as where to get a local SIM is basically flawed.

Yes, this is the big gotcha, the fact that it's timed. Last time I was on a boat (Princess Cruises)I think I prepaid something like $99 to get a slightly better rate, but ended up getting virtually nothing for it because it was so slow. It was taking me several minutes just to log in. It would be so much better to pay by the MB, then even if it costs an arm and a leg you know exactly what you are going to get for your money.

PS Can someone tell me what I did to turn the words "it costs" above into a green link ?
PPS Now I've done that edit it's gone away again. But I get lots of green links in other people's posts and don't really understand why they are there

Another tip. A colleague of mine recently went on a cruise to Norway and took his tablet with him. He didn't use it on board but when onshore he found a wireless cafe and logged in there. Unfortunately, as soon as he connected his tablet started installing a major software update, and he couldn't use it. So make sure you switch your autoupdates off.