In vanilla Doom, water is just a more-or-less convincing floor texture. Heretic improved it a bit by adding flat-specific floor clipping and splashes, which ZDoom manages through the TERRAIN lump, but this still only covered very shallow water.
Vanilla deep water
A number of editing tricks have been developed by mappers to create deep water effects. By increasing degree of hackiness, we have:
- Flat bleeding: by exploiting a glitch in the original renderer's "flood fill" algorithm to draw flats, it is possible to hide a sector's floor under the floor texture or a higher sector that encircles it if the boundary linedefs do not have lower textures. The effect is used notably in MAP02 of TNT: Evilution.
- Self-referencing sector: another hack in which the boundary linedefs are edited to have the inner sector on both sides. The outer sector defines the appearance but the inner sector defines the effect.
- Superposed sector: two sectors with the exact same shape are created, one of them (the bottom one) cut in two by a diagonal and untextured linedef. After they are finished, the two sectors are moved so that they occupy the same space, but the map editor should not merge vertices or linedefs.
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. These tricks remain purely cosmetic; as far as ZDoom is concerned, such constructions are not genuine deep water. They will not affect an actor's waterlevel and will not trigger underwater reverberation.
Boom deep water
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. In the Hexen format and UDMF, ZDoom translates it to 209:Transfer_Heights and adds several optional enhancements as well. Contrarily to the vanilla hacks, it is even possible to make water deep enough to swim in it. This is generally the recommended method to use for ZDoom projects.
3D floor water
Another alternative feature for creating deep water is using a 3D floor for it. The advantage over the Transfer_Heights method is that the water can be translucent, allowing to see in when you are outside; or out when you are inside.
A sector containing a WaterZone thing is considered swimmable and underwater. Placing such things is not needed when using Transfer_Heights or Sector_Set3dFloor, since these specials allow to create swimmable deep water directly; but it might be used for example to create an underwater tunnel between two deep water pools.