...it looks like from other posts that you should have no problem with the IPhone working onboard, and you can also put it in Airplane Mode I think to help avoid the Data Usage charges. But, I honestly can't speak from experience seeing as how on our last cruise on the Wonder my husbands IPhone went "swimming" in the Mickey Pool after our DS (he was 4 at the time).
Long story short, this was DS first cruise and was so excited to go down the slide, that once he got to the bottom and stepped into the pool portion, his feet slipped out from under him and he freaked out so DH had to go in after him. We had let the kids swim as soon as we boarded and DH didn't have his suit on so, everything in his pockets got soaked (including the pager from the clubs).
Needless to say, the phone and pager were toast (or at least we thought so) and we had to report to the club what happened and they were going to charge us I think $300 for the pager, but in the end it wasn't on our bill (Whew, what a relief). We tried and tried to get the phone to work and sat it upright to drain the water out to no avail.
But....great testament to the IPhone....we got home, he turned it on and all he had to do was sync it again and it still works to this day, I know because I use it now that he upgraded to the newer version!
We learned that the cool thing about IPhone is that it automatically shuts off when it gets wet so that the internal stuff won't get ruined!! WE are now sold on Apple!