If you used the wifi in your cabin, the charges would be applied to your onboard account, so it's possible you were charged and didn't realized it because the use of wifi anywhere on Carnival or RCCL is going to have a charge associated with it. Same holds true for cell phone service when out to sea, but that could have been charged by your service provider.
Personally, we like to take our netbook or android into port and do all our business there. We usually find a free wifi bar and have a beer while catching up on emails.
Unless you're close to shore and can get a signal from land, any ship-to-shore communications when out at sea will have some type of charge associated with it and it's usually not very cheap.