To come by bus:

Belize City to Sarteneja - buses to Sarteneja are light blue and their “terminal” is next to the swinging bridge, next to the water taxi terminal. It will cost you BZ$10 to Sarteneja.

Orange Walk to Sarteneja - the bus stop is in front of the San Christopher Hotel, just before the bridge. BZ$5 to Sarteneja.

To come by the Thunderbolt boat:

Corozal to Sarteneja - departs from the Corozal pier to go to San Pedro, stop at Sarteneja on request, BZ$25.

San Pedro to Sarteneja - stop at Sarteneja on request, BZ$45.

Map of Sarteneja

(click on image to enlarge or print)