The easiest way is to get a water taxi from Chetumal to Ambergris. Get an early start from Playa because it's a 4.5 hour bus trip on the express bus, and you need to be at the pier at least 30 minutes early for Mexican immigration processing. Belizean immigration & customs are handled when you arrive in San Pedro.
There are two water taxi companies providing service between San Pedro and Chetumal (see websites for schedules & fares):
a) San Pedro Water Jets International:http://www.sanpedrowatertaxi.com/index.html
b) San Pedro – Belize Express:http://www.belizewatertaxi.com/
Trip time: 90 minutes.
In Chetumal, water taxis use the main pier, “Muelle Fiscal”, which is close to the big Government building, “Palacio Gobierno”, at the waterfront. On my BelizeBus site, there is a map of Chetumal that shows the locations of the ADO bus terminal and the pier. The pier is also visible on Google Earth.http://belizebus.wordpress.com/
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, on the north side of the city, to the pier on the south side of the city. A taxi should take about 10 minutes, and cost about MX$15 (that’s pesos). 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.
The other option is getting a Belizean bus from Chetumal to Corozal (BZ$2), and then getting the water taxi. However, the water is currently running only once per day in each direction -- it departs Corozal at 7 am.
It's about 12 km from Chetumal to the border crossing, and another 12 km from the border to Corozal Town.
Buses & taxis are generally very safe in Belize and Mexico. I recall there was one bus robbery last year -- very rare.