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Some of that already exists, check out the App Store. I'm thinking an iPad would make a decent chart plotter, though :-). Now, if I could only find a weather-proof enclosure!
I have TideApp, but I haven't seen anything like weatherfax on the iphone :-) You're right that chart plotter on the ipad would be pretty tops. As waterproofing.. you'd have to use a material that lets the capacitance of your finger through.
The other issue is connecting a GPS to the iPad if the internal GPS is not good enough. I don't know if it is yet, will take my iPad sailing this August.
Apple requires you to build external accessories specifically to work with iOS devices. I think that even requires a special chip. An external Bluetooth GPS would likely be power-hungry.
I think http://www.tacktick.com/ are an awesome example of low-power marine electronics. They designed and built their own low-power wireless network to connect their instruments and powered them with solar energy whenever possible.