I have never taken the bus all the way down, but there are several threads on this board that describe the trip. As Yoda would say, "use the search, Luke".
If you're tramping around, I'd recommend that you take the bus from Cancun to Playa del Carmen and then grab the ferry to Cozumel. Really, why would a diver hang out in Cancun when Cozumel offers better diving? There's also good diving on the coast of the Yucatán, in the Cenotes and offshore. If you're adventurous, dive the Banco Chinchorro atoll. Then, get back on the bus and head down to Belize. I recommend the ADO bus line, but others are probably as good or better.
I don't believe there's any shore diving at Ambergris Caye. Every place I've seen that was close to shore was
- Overrun with boat traffic
However, I recommend the dorkeling.