| Just returned from a little bit of sailing on a bigger inland body of water, and kept my Geiger counter below deck with history saving every minute. Here is the result (readout with GeigerLog):
When on land the background is CPM=19.4, while over water is is only CPM=13.3. The altitude of the water is at sea level (actually, 1 foot below), while the land is at ~80m. So, the reduced background over water could be influenced a little bit from reduced cosmic rays, but I think the majority is due to the lack of nearby soil, rocks and building materials when on a boat consisting mostly of plastic, wood, aluminum and steel.
As the instrument background is specified by GQ as "<0,2pulses/s", equivalent to CPM=<12, almost all residual background might be originating from the instrument, and almost nothing from cosmic rays.
Though the "<" sign could mean anything from an instrument background of 0 to 12.