The stage is covered in 2-3 inches of water; the characters completely ignore this although it has interesting effects on their movement and of course clothing (they are usually thoroughly drenched by the end of the performance). Pina Bausch (German Expressionist choreographer) did something similar for one of her large-scale dances in the 90s.

Source: Gendai Engeki 60's-90's, p. 46