GPS reception is independent of 3G or WiFi. All you need is an app to record and display your position. iNavX should be particularly useful as you can store maps locally on the phone. Here are some excerpts from the FAQ at
http://www.inavx.net/
"Are the marine charts and maps stored locally on my iPhone or iPod Touch? Yes.
How much storage space do the marine charts and maps take? Depends on how many charts and maps have been downloaded. Individual charts and maps can be deleted. Typical chart or map takes under 1 MB of storage.
Does iPhone or iPod Touch need a cell or wifi internet connection to use the marine charts or maps? No. Only an internet connection is needed to download selected charts. Once charts are downloaded they can be used at anytime regardless of internet connectivity. Note: The charts do not expire.
Does iPhone 3G need a cell or wifi internet connection to use the built in GPS? No. If present, wifi or cell can assist the GPS receiver, but it is not required."
