Even if you bring your own laptop you still have to pay per minute or buy a package of minutes. Per minute varies between cruise lines .50/.75 . A package on Carnival is 100 minutes $55. 250 minutes $100. There may be an activation fee. You will have internet and email capability. If your laptop doesn't have the wireless card you can rent a card for $10 a day.
Some of the ships are also starting to offer ship wide WiFi so you can connect from anywhere on the ship.
These connections can be rather slow. If you just want to check email or surf around it works fine. When I tried to upload pictures to my website the connection would not handle it.