If arriving in Chetumal at the ADO bus terminal, there is a little booth inside the terminal where you can get water taxi information. There is (or at least there once was) a water taxi sign and a person who speaks English. You could probably buy a ticket there, but you might get pushed into an overpriced taxi. I’d just go outside to get a taxi, and buy a ticket at the pier.
It’s about 3.5 km from the ADO terminal to the pier. A taxi should take about 10 minutes, and cost about MX$15 to MX$20 (that’s pesos), and certainly no more than MX$30. The taxi driver might not know about the water taxi, but he/she will know the pier. Write it down if you can’t pronounce it.
When daylight saving is in effect in Mexico, SP Water Jets departs Chetumal at 3:30 pm Mexico time (2:30 pm Belize time), and SP-B Express departs Chetumal at 4:30 pm Mexico time (3:30 pm Belize time).