I have an XM Radio and take it on the boat. It is never exposed to the elements though. You can mount it in the cuddy the only thing that needs to be exposed is the antenna.
They do have a marine antenna that you can buy to go with your radio. I have one of those.
There is a new XM that is for weather only. It is called XM WX. You will actually be able to see the weather on a screen. This is different from the music XM radio.