This is a solid idea, and a fun idea to work with, coming across empty jars or seeing unresponsive people gathered around evaporating water dishes.
although the imagery of a man briskly rubbing a flask/jar with his hands causing it to shoot spouts into the air that then transport that person out of their body....it just made me feel unclean. Maybe that is just me
I like Melvin's twist - occasionally the entire boat is taken as an offering :)
And yes, there is something in the water - perhaps a dragon or similar beastly. It would have sorcerous powers so it would occasionally gift good luck on the donors.
Lots of plot hooks can come from that - a jealous nearby town might try to end the local town's luck by any number of ruses - perhaps poisoning the lake, sending warriors to kill the benefactor, dropping trapped gifts to enrage the beast, etc.
Even nobels could get jealous and seek to know the truth, and perhaps capture the beast.