what plan do I need to use internet on my iphone while on a cruise that supports wifi?
More than likely internet access via wi-fi on the cruise ship will not be free. The cruise ship will have rates for wi-fi access that you can choose from.
Do I need to change any settings on my iphone while on the cruise?
If you don't want to receive or place any calls at international or cruise ship roaming rates and the same for SMS/MMS and be charged international roaming rates for data access via the cellular network, you can place your iPhone in Airplane mode followed by turning on wi-fi access. All other iPhone radios will be off except for wi-fi access on the ship.
Will 3g be covered under my normal ATT iphone plan?
No. International roaming or cruise ship roaming is extra for calls, messages, and internet access.
http://www.wireless.att.com:80/learn/international/roaming/international-roaming .jsp?wtSlotClick=1-0021N8-0-1&WT.svl=calltoaction