You can usually buy the sims on the ferry, a few years ago I bought a Phone including £20 credit for about £25 because my latest model superphone was difficult to unblock. Just remember that when you phone home on a payg sim its an international call and the EU free roaming doesent apply cos the IOM aint in the EU so its down to each individusal operator, T mobile reamed me out a few years back because I misread their tariff £169 for a few brief calls out and my share of the incoming calls from timewasters trying to sell me PPI claims!!!