Rik, you got the info on what I believe is the only Bluetooth GPS that will work with iOS - the Dual XGPS. A normal Bluetooth GPS will not work - it has to be a special one that Apple has approved. I don't know why this is necessary but you just can't use a normal Bluetooth GPS without Jailbreaking your phone/pad and I wouldn't suggest that any longer.
The Bad Elf was mentioned. That is a device that will work with an iPad but unfortunately, it is not Bluetooth. It is a device that plugs into the docking slot. That's not a bad solution for a boat since you'll probably be plugging in the iPad anyway.