I have seen water in LPG tanks in the past, more from a build up of moisture and condensation in a tank, and one of the reasons that what I refer to as "correct" LPG tank filling is done with a scale, (by weight), and not just by watching the stream of fuel from the vent (by a volume guess).
An empty tank has a certain weight, and a tank full of gas will weigh differently than a tank with water in it.