I did it during the summer (and wrote about it on my blog) and am actually going back next week!
Easiest is definitely the bus from Tulum to Chetumal (it's about 3 hours) and leaves frequently.
In Chetumal, you can either take the water taxi directly to San Pedro. (Once a day and about 2.5 hours)
Or, you can take a taxi across the border and fly from Corozal to SP. I believe Maya Air is still doing the discounted price so it is only $49bzd for the flight.
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