driving around Yucatan and into Belize?

[Raksol]

Planning a two week trip for a few months from now. I`ve been to Cozumel, and dove the mainland from Akumal. I love the area, but I`m a bit tired of being a tourist and flying in, going straight to my hotel, and not going anywhere until it`s time to fly out. I want to explore the Yucatan a bit more, and I would love to include Belize in that trip.
Has anyone tried any of the public transport in this area? I know there are actual busses (coaches) as well as the combi`s (VW vans) that locals use. Are either of these a feasible way for 4-8 scuba divers to travel around with all their gear?
I know I can rent a van in Cancun and drive it around Mexico. Has anyone tried to rent a car in Mexico and then drive it into Belize? Good or bad idea? It looks like if you were to follow the coast for about 100 miles south of Tulum, you`d be heading straight for Ambergris Cay. Problem is, my map doesn`t show a road going along the coast after 407 (I think) turns inland. I`ve got a better map on order, but is there a road there? Is there a bridge or ferry across to the Cay? Is there any sort of public (cheap) land transport that will take us and all of our gear across the border and into Belize?
We`ll all speak at least a bit of Spanish, and will be up for a bit of adventure if necessary, but how safe will this be? I won`t mind paying an occasional $50 bribe here and there (split among 4-8 of us shouldn`t be a big deal) but I really don`t want to get all of our gear stolen, or have anything worse happen.
We`ll be trying to spend the bulk of our vacation budget on the flight in, and the diving. Looking for tips on how to get around with tons of gear on the cheap (as well as eating and lodging on the cheap). We might have to limit ourselves to Mexico on this trip, and actually fly into Belize next year, but if we could do both in one trip, it sounds like it would be tons of fun...

[opaque]

Apparently, that is not quite as big a problem as it seems on first blush. If you can get to the border you can take a cab to Corozol. Now from Corozol you catch the Thunderbolt water taxi directly to Ambergris.