THE INSTRUMENT RATED PRIVATE PILOT DEPARTED IN SPECIAL VFR CONDITIONS FOR A FLIGHT TO ANOTHER AIRPORT ABOUT 17 MILES TO THE WEST. HE WAS ADVISED TO MAINTAIN VFR. HE RECEIVED VFR ADVISORIES FROM ATC, WAS GIVEN A VECTOR FOR THE DESTINATION AIRPORT, AND AGAIN ADVISED TO MAINTAIN VFR. ACCORDING TO THE PILOT, HE ENTERED A FOG BANK AS HE NEARED HIS DESTINATION. HE SAID HE INADVERTENTLY LOST ALTITUDE WHEN HE ATTEMPTED TO ADJUST HIS GLOBAL POSITIONING SYSTEM (GPS). HE TRIED TO REGAIN ALTITUDE, BUT WAS UNABLE TO RECOVER IN TIME TO AVOID HITTING TERRAIN.