Hi!
I'm from Holland and I'm an exchange student at the UNAM in Mexico-City. I visited San Blas 2 weeks ago. I'm not quite sure if Orange-fronted Parakeet occurs in San Blas, but according to Steve Howell's 'A Guide to the Birds of Mexico and Northern Central America', San Blas is part of Orange-fronted Parakeet's range. However, things changed rapidly over the last ten years, which is mostly a bad thing, but partially a good thing, since species such as Snail Kite and Limpkin have settled in the region and now occur frequently, they're actually quite common.
I would highly recommend you to ask for Chencho when you're in San Blas. The 'A bird-finding Guide to Mexico' you're about to buy describes detailed information about San Blas, and Chencho is even mentioned as a recommended boatman. If you're heading towards the dock for the mangrove boattrips and someone offers you a tour in a lancha, ask for Chencho and your tour will be a lot more interesting. He's an older birdwatcher who loves to guide you around in the mangroves with his lancha. That way you will see some species you would never observe on land. For example, I spotted a Northern Potoo! Also, Chencho taped Colima Pygmy-Owl, and inmediately after the tape, there where Colima Pygmy-Owls all over the place bringing along (Mangrove) Yellow Warbler and other American Warblers. We also spotted Magrove Cuckoo, almost all species of herons, Snail Kites, Ospreys, (Mangrove) Common Black-Hawks, Purple Gallinules, Green and Belted Kingfishers, Golden-cheeked Woodpecker (endemic), Lineated Woodpecker, all kinds of tropical flycatchers, Yellow-winged Cacique, the endemic Happy Wren, Sinaloa Wren and Sinaloa Crow and many species more.
If you have a car, you should follow the instructions in the 'A bird-finding Guide to Mexico'. Unfortunately I didn't, so I guess I missed a lot (I couldn't go to the Singayta for example). Of course, San Blas also has changed recently, and not every detail is correct anymore. But check the description of the Sewer Ponds Trail. I was lucky enough to see the endemic Mexican Parrotlet there. Along the track you will also see little lakes / lagunas which are extremely good for water- and shorebirds. At the end of the track you'll find two estuarios, a sort of ponds, where you can see Mangrove Swallows, Yellow-winged Caciques and Teals.
Another good place are two larger ponds just outside the village, used by shrimpcatchers. If you leave San Blas, there's a road to the left. After a few hundred meters you'll see a gate that leads to the ponds. These ponds are extremely good for water- (mostly herons and kingfishers) and shorebirds.
For Brown Pelican and Booby's just go to the little harbor of San Blas, they're pretty abundant. Playa El Borrego next to the harbor is an easy place to spot Black-bellied Plover, Willets and Spotted Sandpiper while relaxing underneath a palapa and drinking your agua de coco!
Of course you will visit San Blas during another season, so not everything I mentioned might work out, but I hope this might help you a little.
Joren