What cruise?
Eastern--
St Thomas--walk toward the cable car ride entrance. Just before the ticket booth is an Asian restaurant. Wifi is $3 for all day, less if you buy a meal. LOTS of CMs there. There is wifi in a bar in Havensight mall, but we didn't like the way it looked--no one around--so we went to the place the CMs suggested
St Maarten--McDonalds, free
Western
Cozumel--Starbucks at the port, free
Grand Cayman--haven't used it there for years, so I'll defer to someone with more current info.
Key West--lots of places, $5 for one hour We found a couple just by googling it.