There are no antennas that I know of that are capable of recieveing both GPS and XM, but In all the boats we do we never place the antenna where it is visible. In my boat I have tha XM antenna stuck to the top of the radio under the dash, and it never cuts out unless someone is sitting on the bow directly above the radio. Same thing with the GPs antenna It shouldn't have to be mounted on top of the fiberglass. In my truck the GPS antenna is mounted in between the headliner and the roof and works just fine.