Does the Create Water spell need to be cast on a vessel that would be filled by it?

Question

The Create Water spells reads:

In an open container, you can create up to 10 gallons of drinkable water. You may also produce a rain that falls within a 30-foot cube and extinguishes open-air flames.

Must the container be filled by the water created? IE, a horse trough or sinkhole would not be valid targets, since they can hold more than 10 gallons of water?

If not; what prevents me from simply choosing the room I'm in as the container? For example, creating 10 gallons worth of water on the floor with the intent of freezing it to make ice terrain?

Or further, choosing the room and creating a gush of water over an unruly bar patron to "cool them off" - I can create rain, does something restrict me to Creating Water on or very close to a surface?

Answer 1

I believe the answer to your question may lie within the spell's written affect, You can create up to 10 gallons of drinkable water.

There is nothing that specifically states it must fill the container, but something that does state you can create up to 10 gallons of water (as in you can create between 0 and 10 gallons of water). So long as it can be an area with an open space, you should be able to create water inside of it.

Note: the GM has final say, given they have more power than the gods in their universe.

Answer 2

Must the container be filled by the water created?

No. The relevant portion of the create or destroy water spell description states (PHB 229):

You create up to 10 gallons of clean water within range in an open container.

There is no mention of the need to fill the container.

If not; what prevents me from simply choosing the room I'm in as the container?

Simply that a room is not a container. A room is a room, and a container is a container. There's no reason why the word container would mean room in this case. If you want water in a room use the rain feature of this spell.

D&D 5e generally uses words to mean what they mean in everyday English unless otherwise stated. Here is a tweet from Jeremy Crawford, lead rules designer of D&D, to support this:

Unless the rules explicitly expand, narrow, or completely redefine a word, that word retains the meaning it has in idiomatic English. #DnD

I can create rain, does something restrict me to Creating Water on or very close to a surface?

Yes, the text of the spell description clearly states, "in an open container." This means you can create the water in an open container. Why? It's D&D, that's how the magic works.

All that being said it would not break the game to expand this first level spell a bit and allow water on the floor of a room. Of course, your GM would have to approve this usage, as it is not allowed by RAW.