The foils you were finding could be a couple different things, if they had black residue on them they were probably being used to smoke either heroin, or in some cases meth, but foil can sometimes be used to package drugs as well if there was no burn marks on it. As far as a jar being filled with water, that is odd. Sometimes baby jars are used as a pipe for crack
cocaine, but water is normally associated with pot smoking, or slamming certain drugs. If he is loading a syringe he would be using water, regardless when he dissapears into the bathroom, he is up to no good it sounds like, this is coming from a former addict of many things, hope this info helps you a bit.