Making deep water

Deep water is an effect that allows a level designer to put false floor textures on a pit or trench in which the player can sink. If the pit is not very deep, the false floor texture will still be displayed after the player falls, simulating swimming or the player crawling on the floor.

Implementation
There are several known methods to obtain this effect. Some of them work on many source ports, while others only work on vanilla Doom. Also, some of these methods might not work on certain level editors.

Vanilla

 * The very simplest method is by exploiting flat bleeding and surrounding the deep sector with a shallow (higher floor) sector. The boundary linedefs between both sectors are left without lower textures so that the shallow sector's floor bleeds over the deep sector and hides it. This method is easily detected and emulated by hardware-accelerated source ports and can be used with any map editor. It is used in MAP02 of TNT: Evilution.
 * Deep water can be simulated by using self-referencing sectors. This method is the most common, and some level editors like XXXXX already include them as a feature.
 * Deep water can also be obtained by superposing sectors of identical shape, tracing a diagonal linedef on one of them.

Since these methods remain very limited and prone to glitches, and may not be well supported by all ports, nodebuilders or even map editors, advanced ports started defining genuine methods to create deep water.

Boom
Boom defined several property transfer linetypes to achieve effects not possible before -- or at least not possible without hacks. The one meant to create, among other things, deep water, is linetype 242.

ZDoom (Hexen)

 * Linetype 209:Transfer_Heights, optionally with thing 9045 (WaterZone) to set friction.