The excellent NAVIONICS for iPad (also available for iPhone) is probably the best Marine Charts Navigation system available for a mobile device such as the iPad. 

Note that only the iPad WIFI +3G has a GPS chip! Quiet important as even if charts are preloaded a real GPS is required to fully use this App.

I think apple should have made it clearer to consumer, much more +3G would have been sold if people understood that their WIFI only iPad is A-GPS (Assisted Global Positioning System) which use an available WIFI connection to find out where the device is located, and not GPS which require a CHIP.

They probably thought a real GPS would be useless without 3G connection. Yes but, more and more NAV Apps use pre-loaded maps and charts.