Does it exist? More importantly, since I'm pretty sure it doesn't, does anyone want to do it?
The reason why: I'm working on yet another mod...or 2...or 3...but the one in question is a Sci-Fi colonization mod, and one of the 6 planet types (the other 5 I can accomplish through recoloring of existing terrain...) will take place entirely underwater on an Oceanic planet with no 'land'. Instead, continental plates will form what would be described as immense Guyots, with "underwater oceans" of sea floor between them.

Such a terrain style would use the following designations:
Grassland: Sea Floor (nothing special)
Plains: Lush Sea Floor (dotted with miscellany flora)
Desert: Sandy/Barren Sea Floor
Tundra: Glacial Sea Floor (cold-looking sea floor...not sure how this would be accomplished)
Forest: "Kelp'd" terrain (with Kelp-like forests)
Jungle: Coral Biomes (low, on sea floor: hill and mountain with 'jungle' could potentially look very interesting)
Hills: Rough Sea Floor (just low, craggy terrain)
Mountains: uh...Mountains? Pretty much same as civ mountains, just craggier to suit the underwater environ.
Volcanoes: (same as above)
Marsh: Thermal Vents
Rivers: Continental Ridge, perhaps? Not so sure for this one.

All "Land" terrain would be deep green/blue, with all "Water" terrain being brackish black/blue. I would envision a 'beams of light' effect on the "land" type terrains, ideally.
If there aren't any takers, I'll probably end up trying at it myself with recoloring and cut/pasting.
There are plenty of interesting applications for such a terrain set...a postapocalyptic earth mod where people live underwater, a fantasy mod with merpeople or 'atlanteans', or any number of scifi mods.

I would love to see terrain like this made. :)

Convert it from civ2 Test of Time.

That was my plan until I re-discovered that my Test of Time disc had been stepped on...

Check the civ II forum here or at apolyton, I downloaded a good underwater mod for that a year or two ago, you should be able to get some good ideas from that.