The GPS is just a bonus. Don't worry about having sonar only. The GPS in most of the HB Helix units are all with built-in antennas, so there is nothing you need to worry about mounting. Look at the Helix 7 series. Excellent units and has a great screen.
Capt. Craig Copeland
Licensed by the US Coast Guard
2013 Nautic Star 2110 Shallow Bay Boat
Humminbird HELIX 12 CHIRP MEGA SI GPS G2N
Nautic Star Boats Pro Staff (nauticstarboats.com)
Redneck Fish'n Jigs Pro Staff (redneckfishn.com)