Well I have do not know about the water in the propane tank scam, but I do know about the follwoing:
The gofors, your guys on the moto or pickup, they will take your tank, 100 gallon tank will only fill with 80 - 85 lbs, or less depending if they are a continueing scam or a one time deal, the regulars will only short you a gaollon or two each time the one on, may short you 1/2 the tank, if you are woman or a small guy, or older coot, well they will take even more advantage. At 98.9 pesos a gallon, it is know very lucrative business.
All this come from a propone delivery guy I know, I have always filled my own, even then I know thge guy at the station ripped me off when he said he could fill a 6 gallon tank with 7 !
We use the colmado delivery service here for the beers and brugals etc., occesionally the misses wants pica pollo, of course, so she asks the delivery boy from the colmado, to stop by the pica pollo and bring dos servicios de 5 pedazos, right, this 10 pieces feed all five of us no problem, but trick motoconcho, asks pico pollo guy to pica el pollo, he bought 3 piece pica pollo chopped up in pieces and sold to the misses as 5 pieces, whoi could tell, right, well....
They will take your shirt, underware and socks if you are not careful.
g'luck