Q- Do I still need a marine compass if I already have a GPS onboard?
A- Even if your boat is already equipped with a GPS, there is no substitute for the marine compass.  If you are venturing for more than one mile or two from shore, it is advisable that you have a compass onboard even if you have the latest in electronic gadgets.  Navigation electronics are advanced but they can never replace a properly compensated and adjusted magnetic compass in the event electrical power to the electronics is lost.
Marine compasses are relatively inexpensive, with some quality brands starting at $36 dollars as the Ritchie X-10-M-CLM COMPASS but can go as high up as almost $900 with the RITCHIE SP-5C (STAINLESS) 5 DEGREE 12 VOLT seen below.