Water is probably one of the easiest effects you could ever make.
To make waist-deep water which the player can wade through, first make a sector and give the floor a water texture (#336). Now tag the sector [0,1]
. That's it! If you want the water to be green like sewer slime, just give the water texture a palette of 8.
Note: Any sector with a Lo-Tag of 1 will act like water, allowing the player to float waist deep (even if the floor has a regular floor texture!).