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The control water spell has the option to separate water and form a trench on a cubic area of up to 100 feet per side:

Until the spell ends, you control any freestanding water inside an area you choose that is a cube up to 100 feet on a side. You can choose from any of the following effects when you cast this spell. As an action on your turn, you can repeat the same effect or choose a different one.

[...]

Part Water. You cause water in the area to move apart and create a trench. The trench extends across the spell's area, and the separated water forms a wall to either side. The trench remains until the spell ends or you choose a different effect. The water then slowly fills in the trench over the course of the next round until the normal water level is restored.

This is easy to imagine when cast at the water's surface. There is air above the water, and so parting the water forms a cube-shaped trench with breathable air.

What happens if the targeted area is all underwater, not near the surface? Does parting the water create a cube of air?

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    \$\begingroup\$ Highly related: What effect, if any, do water-creating spells have underwater? \$\endgroup\$ Aug 5 at 20:14
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    \$\begingroup\$ quite possibly one of the best questions ever asked on this site. 😉 \$\endgroup\$ Aug 5 at 20:22
  • \$\begingroup\$ I see two possible issues: 1) an area that is completely underwater is not 'freestanding;' 2) a 'trench' is different from a bubble or void. Under a strict interpretation I think the spell would fail. \$\endgroup\$ Aug 6 at 21:18
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Who's to say?

The DM, that's who. Naturally, when the trench created opens up to the surface, the area is filled with the air from the above the water. But when the trench created is entirely underwater, the description gives no guidance as to what happens. Does it created a vacuum? Does the water column above the void drop down, filling the trench? Does it create a pocket of breathable air? The spell description does not give anything with which to begin answering those questions, so the DM needs to make a ruling.

If you have control water, and anticipate going on an underwater adventure, talk to the DM ahead of time. Have a conversation and work out together what the spell is going to do, so that your expectations are not squashed when the DM rules differently than what you thought. If you know a particular spell interaction has potential for confusion, work with your DM ahead of time so they don't have to make the ruling on the fly. And if it comes up, don't be afraid to have a review conversation with the DM. Did this ruling work well? Should we use this ruling if it comes up again?

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    \$\begingroup\$ As an extension to this, if you're the DM, don't try to overthink it, and definitely don't try to find a way to make it consistent with real-world physics. You won't succeed, and you'll most likely just end up with exploitable (and still inconsistent) mechanics that let your players create magical depth charges or something similar. \$\endgroup\$ Aug 5 at 19:48
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    \$\begingroup\$ @IlmariKaronen I am the DM in that case, and we both went "hmm, I wonder how this works?" so I went with "OK, there's no water there until it stops being parted" since I'd never used that monster/NPC before in a purely underwater fight...and I wanted to see what happened if I did that. A fire bolt and a produce flame landed and didn't have the damage halved: that's what happened. Mostly. (This past Monday night). Mike and I will be reviewing that before we play again, since it made us both scratch out heads). \$\endgroup\$ Aug 5 at 20:18

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