It is quite possible and cheap to go by bus from Mexico into Belize and even Belize to Honduras. But that second part is long and tedious. Plus, you miss the beautiful islands of San Pedro and Caye Caulker. This is a viable route, the route I took.
From Chetumal, Mexico to San Pedro or Caye Caulker, Belize (same boat) by water taxi. (I selected the green and yellow boats, Belize Express Water Taxi, and was later told it was the most reliable of the two companies.) (Be sure to discuss the price. As more potential passengers arrive, The show,of competition is interesting. When Spanish speakers were given a better price than I was quoted I was given that better price. I was also able to pay by credit card.)
From San Pedro or Caye Caulker (same boat) to Belize City by same water taxi. (I bought the ticket at the same time as the previous one, saving money.)
From from Belize City to Puerto Cortes, Honduras. take the Pride of Belize boat (Only Saturday 9am, Belize $145. Cash only, no credit cards. Goes from the Radisson pier, an easy walk or 5 minute cab ride from the Belize ferries.) This boat stops at the Dangriga pier and immigration comes on board to check you out of Belize. Fee of $7.50 Belize dollars.
Alternative boats from Belize to Honduras:
From Dangriga (out if commission for a while April 2012
From Placencia – (Leaves from Shell Dock) Every Friday at 9:30 a.m.
And from Big Creek Every Friday at 11:00 a.m.
Approximately $US 55 (110 Belize Dollars)
That’s the end of the water travel. Almost…depending….
Puerto Cortes to San Pedro Sula by bus. That part isn’t fun. At least, the bus we found, which was a step above a chicken bus wasn’t fun. But it was ok. We had to walk from the ferry landing over a bridge to catch the bus.
If going to the Bay Islands, either Roatan or Utila, you need to get a ferry from La Ceiba so San Pedro Sula La Ceiba by bus. Then the Galaxy ferry from La Ceiba to Roatan for US$27.
You can also take a bus I am told is great to Copan Ruinas, Guatemala from San Pedro Sula.