You will incur the same charges to connect your Iphone to the ship's WiFi as you would a computer. It is .75/minute if you pay by the minute. $100 for 250 minutes and $55 for 100 minutes. The speed is not what you would get at home unless you are still on 56Kbps dial-up.
I would also advise you to turn roaming and data off. If you have roaming and data turned on your Iphone will connect and you will be charged for data usage and any calls either incoming or outgoing. The voice rate is $2.95/minute.