I myself have contemplated this idea, but as DX pointed out, 15 to 20 feet is very weak.
Typical store-bought soakers achieve ranges of up to 35-feet, the CPS-2ks will often easily hit 50 and the maximum range for homemade beam-weapons thus far is 73 feet.
Then if you were to delve into the water-balloon-laucher line, you'll see ranges of up to 500 feet, though they are not particularly accurate at this extreme distance.
As a trap, it
might have some potential, but not as a main weapon.
I believe however, that Buzz-Bee-Toys did in-fact manufacture something like what you are describing here:
http://www.isoaker.com/Armoury/soaker_stats.cfm?soakerID=wwsplatblaster