When I was playing versus online with a friend on the Roasted Resort map, we discovered that if you walk into the pool with an already cleaned glass it will spawn back as a dirty glass.
The easiest way to get this done as fast as possible would be to make a drink consisting of 3 ingredients and deliver it. Make sure to deliver the drink to the correct side, red team should deliver on the red exit and visa versa. After you have delivered the drink it will spawn back as a dirty glass, rinse the glass with the water gun and give it to your other character on the other side of the counter. Switch to that character using the
button and walk with the newly cleaned glass into the water. The glass will respawn again after 5 seconds in the dirty glass crate, rinse and repeat. Doing it this way you can easily get up to 15-20 glasses per round.
This also works offline in couch versus. However it doesn't work with multiple glasses at a time, since the 2nd glass dropped in the pool will just spawn back as a normal glass.