Dan, as you have seen, Winlink is often used on boats at sea, for email
and weather information (search "GRIB" for details on getting wx info while at sea).
As mentioned, Sailmail, and a few other organizations, provide Pactor radio email services as an alternative to ham radio Winkink. Sailmail is a nonprofit, founded by sailors, and costs $250/year to join. Sailmail has a limit on the weekly connect-time, and Winlink, being an amateur radio service
, is restricted to non-commercial traffic.
As a lower-speed alternative to Pactor, some Winlink operators are using a free software modem
The Pactor modem
also makes an excellent weatherfax demodulator, letting you directly receive the wfax broadcasts from the worldwide shore stations. There are also free or low-cost software
soundcard demodulators for wfax.
Some people use satphones for email while at sea. Look at Iridium
and Global Marine
Networks "XGate" for an example.
I mostly use the satphone, sometimes Sailmail, and occasionally Winlink. These all have their strengths and weaknesses, and wildly different cost models. You will get strong opinions and good advice on any of these topics here, and you will find many archived discussions on the merits of SSB