Take the meager tip you were going to leave, dollars, coins and all, and drop it in the water glass.
Now depending on your manual dexterity and your mean streak, place a coaster on top of the glass. Holding the coaster in place, flip the glass over quickly, place it on the table and remove the coaster. This will leave your tip in the upside down glass with no way to remove it without spilling water everywhere.
Whats the worst thing that could happen, the manager tells you not to come to the greasy spoon anymore? You were never going back in the first place.