Players want to cast Temple of the Gods on a small floating square at sea. (It involves the shape water cantrip, but I want to put that aside for this question). The spell says it requires to be cast on ground– so right there is a solid argument against allowing this, but I can see the argument for the floating area to be considered "ground" as long as it's afloat. The issue I want clarification on is size of the area, though. Spell says it requires a 120ft cubic space. That, coupled with the "ground" requirement, leads me to believe that a 120 ft square of "ground" is required. However, the ambiguous wording requires a logical leap.
Disregarding "rule of cool," or the fact that we're really stretching shape water– does the area of the ground matter when casting Temple of the Gods?
Best Answer
You can make a very small temple
The spell says:
Since it says "up to" and "must fit within", there is no specific minimum size for the temple. You could conceivably make your "temple" a 5-by-5 "holy closet" occupying a 5-foot square of "ground". I guess you could even make a tiny scale-model temple that would fit in the palm of your hand, although that would not be very useful.
Your DM will have to define "ground" (and maybe also "stationary")
For determining what qualifies as "ground", you're going to need a DM ruling. You also might run into issues since the temple walls are made of magical force, and if your "ground" is a cube of ice floating on the water, it's not clear whether a temple created on top of this ice would move with the ice. Again, a DM ruling is required, as you are running into the well-known issue that the rules don't define a "frame of reference" for what counts as stationary. (This issue comes up fairly often for things like casting AOE spells on moving vehicles, so there are already several questions on this site dealing with it.)