Otto, there is another possible way, but will take a couple of phone calls and some timing.
My wife and I just spent a week on the north end at Tranquility Bay (12 miles up) and it is beautiful. See our report under “Trip Reports – A Week Of Tranquility”:
The water taxi’s last stop going north is Blue Reef, about 8 miles. The Turtleman, whose house is next to Tranquility Bay, takes his kids every day to Blue Reef to catch the water taxi to school in San Pedro and picks them up at the end of the day. Call him and see if you can make arrangements to meet him at Blue Reef:http://turtlemanshouse.com/
Also you can call Tranquility Bay and see if you might be able to correct with one of their boats picking up and guest and/or supplies, but that varies a lot:http://www.tranquilitybayresort.com/belize/index.cfm
No matter how you decide to explore up North, you will not be disappointed!!!! TB has a great restaurant for breakfast, lunch and dinner. Also excellent snorkeling off the beach.