There are two AC-Mainland water taxi services that leave from the eastern shore of San Pedro. They alternate times, so the one you choose to take will depend on when you want to go.
Schedules tend to change. Best bet is to check when you get in and make travel plans accordingly.
The locations - both on the east side of the island:
Texaco pier on front side of the island - CC Water Taxi.
One or two piers north of Playa Lounge - SP/Belize Express.
I would not leave the golf cart overnight on the street - it will probably be fine, but you just never know. Why not a take a regular taxi ride and relax.