Luggage and bags:
Backpacks --- NO big suitcases as road is bad and boats are small. Also bring PLASTIC bags for everything - as the waves can get high or it might start to rain and then you WILL get wet! (you and your luggage). There is nowhere to hide!
Clothing/Shoes/Weather Gear: Sandals is the only thing you need - they will be full of sand! Raincoat for time spent in the boat (if you wanna stay dry).
Summer clothes - beachwear - comfortable clothes for nights (still warm).
Toiletries and Medical Supplies: All medical needs you would have - noting to be bought anywhere.
Photo Equipment: Everything you need - including films/cards - nothing to be bought
Camping/Beach/Outdoor Gear: Might also wanna bring a silk sleeping bag - all you need as it is very humid.
Bring TOWELS (both for beach and shower afterwards)
DONT forget suncream -- minimum SPF 15 I would say or SPF20 better -- cannot buy on islands
Miscellaneous: Only thing we could buy on the island we stayed was these drinks: Beer USD1, UniCola USD1 (bad version of Coke) and Water (stearalized, not good tasting) USD2 - big bottles only.
A small show sold crackers and sometimes they had more variation of sodas. Bring snacks and perhaps a small bottle to put water in (as they only sell big ones)
Bare in mind that San Blas is an island, so don't forget your swimming costume; light clothes are recommended, as most of the day is hot.
Toiletries and Medical Supplies: Mosquitoes repellent, suntan lotion, plasters, towels, etc.
Photo Equipment: Plenty of films...
Miscellaneous: Carry enough bottles of spring water for the journey, as the water in the island is not suitable for drinking.