I maybe sharing a little too much here but... We go to Mexico often, mutiple times every year, and I have a Medical Marijuana card issued from the state of Arizona, so here at home I can go to  dispensery and get pot legally (at least by Arizona law) and I NEVER buy pot in Mexico and here is why...
Mexico has decriminalized the possession of up to 5 grams (between an eighth and a quarter ounce) of pot, along with other things like heroin and cocain. So you could go on the assumption that it's OK for you to have some too. The problem comes along in a couple ways.
First, like stop signs, many laws in Mexico are more of a suggestion than a command. Odds are great that the cops, federales etc could care less and would just leave you alone. There IS a small possibility that a seller will work in cooperation with local cops to sting you. Even though you may have a small amount, any amount is enough to hassle you and the local cops know that most tourist will pay a small "fine" than have to deal with anything. The reason for the sting is simply that being a cop in Mexico doesn't pay worth a crap. Many Hotel workers make more than a cop because of their tips. The stings are a way for cops to get "tips" of their own.
Second is- Under Mexican law you are essentially guilty until proven innocent. This gives the cops great leverage in getting "tips".
For me, Mexico is not a dangerous place overall or we wouldn't go. That being said, buying pot is always a shakey proposition and just not worth it to me. But hey, the chance of a problem is small, and if you happen to buy some and we happen to be at Desire the same time you are I'd be happen to smoke with you ..

P.S.  If you still want to get some just talk to any vendor you see on the beach. They will be able to find you someone to sell it to you.