You will have no problems picking up the sats, it's just a manufacturer's way to cover himself (in the case you mount it on an alu or steel boat).
Your rigid structure is GPR, a fiberglass with resin which will not stop your VHF from a good reception.
I have a sailboat that had a 20 years old Garmin sitting inside, with GPS antennae (on the same bracket as the chartplotter, not outside). The reception is 100%
I changed this one for a Furuno 1670F, with fishfinder