If you leave Playa Del Carmen early enough (like 5 or 7 a.m.), and then take a cab across the border, you can make it to the Corozal airstrip with no sweat. If you leave too late, and then try to do the bus the whole way, border crossing and all, it will be pushing it and you probably won't make it. The 1st class express bus from Playa to Chetumal is 4.5 hours. If you cab it from there to the border, do your crossing, and then switch cabs to a Belizean company, you shave off a lot of time. Speaking Spanish helps, when in Chetumal, negotiating for a cab. I've never paid more than $15 U.S. toi get from the bus station to the border. Once in Belize, you will have to use a Belizean cab company, they don't seem to allow Mexican cabs past the Free Trade Zone.
To be in Corozal for the 7 a.m. boat to San Pedro, you need to leave on the midnight bus from Playa Del Carmen.
I've tried to do the bus the whole way, once, it was frustrating, so now I usually cab it across the border, the crossing is faster.