This may be a common problem that I should know, so if itis, post me a great big DUH and explain it. I'm trying to make a sewege tunnel with a little water in the bottom that has a grate going into a room. The only problem is, I cant see either. the surfaces are there but there is no mat on the surface.
Help me massassians... you're my only hope.

What flags are set on both the surface and adjoin flags?

do you have the flags in the geometry set to "render"

http://www.massassi.net/tutorials/water/tutor.htm
http://www.massassi.net/tutorials/grate/slash.htm

These two tutorials may be of some help. The water ones contain some unnecessary extras for being able to swim, and I don't know whether you want your grates to be slashable.

It does indeed sound like your problem at the moment is that the sufaces aren't flagged properly. Remember that the adjoin will have TWO surfaces. You need to flag the surface that belongs to the sector you will be viewing the adjoin from - in the case of the water, that will be the top one. In the case of the grate, it sounds like both sides of the grate.

I agree with him. Its prolly the geo flags.

Use S(sector) mode and navigate(n=next; p=previous sector) to the sector that touches the water surface, then go into F (surFace mode) and navigate til it highlights the water surface, press enter to bring up the surface menu.

You prolly have the geo=0.

Change it to geo=4.

Check your F12=preview mode again, and see if the texture appears.
( also make sure the texture you have selected is either a jk texture, or if custom-made, make sure its in the \level\mat folder ).

As for your grates(and possibly water?) make sure that if u are using a new custom mat texture, that it is not solid-black. Even if u set geo=4, but have a solid-black mat, it wil not render (or it will render see-thru) just like u had no texture at all.