Is it common practice for hotels in San Pedro to require full prepayment prior to arrival?

Very common. In my 5 visits to San Pedro I have had to pre-pay each time - the last time being this past August.

I prepaid for my first two night in Cahal Pech (jungle inland), and the first 3 nights at Banana Beach. It was so slow, however, that I could have shown up and gotten the room at the same rate (mid October). There are package deals at some hotels (Banana Beach/Mata Rocks) that I checked out, I just wanted the flexibility (and as I said, it was slow).

Hi Adam,
Just to let you know... not all Hotels require pre-payment. We do require a 2 night deposit. In addition, some hotels charge a 10% service charge which we do not.
Cheerz,
Vicki
Caribe Island

It might be for some companies. Our deposits, however, goes directly into the Atlantic Bank account and is reported and taxes are paid.
Cheerz,
Vicki

Just stayed at the Mayan Princess. Only had to put down a one night deposit. However, Mayan Princess does charge a 10% service charge.

The reason why this web site is so awesome is
that you can go into the lodgings section and all the information you need to make an informed choice to stay is there.
I've looked at most of the hotels and they all state what they require as far as deposits, taxes and service charges as well as policies on cancelations. The places I have contacted thru email have all been very helpful and very nice.
If you ask in advance about special prices during paticular times, I feel sure they would advise you.

The reason many hotels and other businesses (not all) charge a hefty deposit, and full payment in the last 4 or 6 weeks, is because of no-shows. I've lost hundreds of $ holding resources for people who just don't turn up.