Not sure how far north Costa Maya is from the bridge north of SP, but I drove a golf cart about 2 miles N this week before I turned around. We did a dive with some people who stayed at Belize Legacy (and they loved the place)who rode bikes to town, and took the water taxi back, way too hard a bike trip and they were in their 30's.
Bikes, golf carts, and street taxi are preferred transpo in the general area of San Pedro and south, if you choose to go N of SP by more than a few miles I think water taxi is preferred.
Wherever you are within reason, dive and tour operators will pick you up. We have had especially good experience with Suya Tours and Carlos the owner. He provides an exceptionally good experience with a great group of locals who really seem to care about showing you a good time. Best of luck with your travels.