I know about XM Radio through my work. Basically, XM has 2 large geostationary satellites (with a 3rd to be launched shortly). They are positioned for transmission to the United States and main application is for radio (music/news) to vehicles direct from the satellites. It is much better quality than FM. They can offer services to boaters as you have seen through one of their many channels.
Unfortunately due to the location of the satellites, there is no coverage for Europe, so broadcasts from the XM Satellites are not avaialble here.