So I contacted my cell phone service provider and received an answer that makes total sense. I am posting it for others' information who haven't been able to find it out. My cell phone provider is Telus Mobility in British Columbia, Canada.
(BTW, I guess I will be keeping my iPhone in airplane mode and just use it for games and music.)
"If you are going to be aboard a cruise ship, your iPhone 4 will have service. The standard roaming rates for cruise ship satellite signal is $7.00 per minute, $0.60 per message and $25 per megabyte. If you wish to keep your roaming turned off, then you would have no signal at all and will not even be able to text. So in order to have signal for texting, you will need to have the roaming turned on.
The cruise ship will forward all charges for usage on your cellular phone to TELUS, which in turn will show up on your next invoice from TELUS Mobility."