It appears that the most southern port to clear out of Mexico on the Caribbean side is Puerto Aventuras or Cozumel, as Xcalak is no longer a port of entry/exit.
Does that mean that we cannot stop anywhere in Mexico after we officially clear out? Do we have to stay on the water until we clear in in Belize?
Sorry if this is a naive question. We're a bit "untravelled".
Usually, as long as your exit papers are in order ( so they know you weren't trying to sneak out ), they don't mind you stopping at Chinchorro or Espiritu Santo or wherever--it's not like there's checkouts there, and they know it. The Navy (who will probably look over your papers) are usually very nice.
Not sure about now but there was an Army base at Banco Chinchorro. Very very friendly guys but they wanted some beers, and sugar for their coffee. We had no sugar and I was low on beers so I only gave them maybe 4-5 for the boat load of maybe 6 guys. They circled around the lagoon drinking beer before heading back to base.