A tip I think I should mention for iPhone users is that you can turn on Airplane mode on the ship, which other posters have said, (since many of us use our phones as calendar, camera, alarm clock, watch, crutch, etc. and want to keep it on and with us on the ship.) but when you are in port, if you happen to stumble upon somewhere with free WiFi, you can switch on only the WiFi without disabling airplane mode.
This will allow you to surf the web, send iMessages (texts to other iPhone users), use FaceTime, post your pictures to facebook/twitter, email family and several other things.
Once you are done soaking up some free WiFi, simply switch the WiFi back off.
You have never touched the airplane mode switch, which means your phone never looked for a cellular signal, never tried to retrieve (regular) text messages or anything like that.
I just recently discovered that you could use WiFi while airplane mode is on, so I thought it would be nice to share, in case I wasn't the only one who didn't realize this.