A belizean water charter can pick you up in Xcalak. The gas alone is $95US, as I just went up for some tacos and beers, for a small speedboat with a 150hp. The transfer will take you aprox 45 minutes.
No mass or public transit at this time.
You may want to pick up a transfer from Mahahual to Chetumal and then into Belize by bus. This would be the cheapest and not much more time. Its all good paved roads from Xcalak to Chetumal.
Swimming is also an affordable option.