OS X (apple) already allows users to share an internet connection (cable, modem, EVDO, etc.) via another port: FireWire, BlueTooth PAN, or AirPort (Wifi).
We did this once on vacation where one computer had an unlimited EVDO card and five other people were able to check their e-mails simultaneously.
I'm surprised that Windows does not have a similar feature.
I'm not sure how mifi could improve coastal cruising internet access unless you were in a club cruise and one of the boats had an internet connection that they were sharing with the fleet. It looks like the mifi needs an existing internet connection.