RAW
Most of this is something that will have to be determined by your group / DM:
The rules don't clearly state whether or not the suggestion itself is the verbal component for Suggestion.
The rules do state that someone who makes a mental save is unaware of the spell unless the spell has a "perceptible effect" (PHB, page 204).
The rules do state that you need a material component for the spell, meaning you either need the component itself, a spell component pouch, an arcane focus, or a holy symbol.
But the rules don't really state how obvious it is when someone "uses" an arcane focus or holy symbol. Only that you must have a hand free to use a material component.
RAI
- Jeremy Crawford has stated that the Suggestion spell requires the chanting of "mystic words" in addition to the spoken suggestion.
A few Suggestions
Here are what I see as the best practices for this scenario.
If you want to take an action, but don't want players to know you're taking an action, you should make a stealth check.
See: How loud/obvious is a wizard casting a spell?.
This stealth check covers stilted arcane language, somatic gestures, glowing arcane foci, etc.
There would be a variety of skills to detect this
Insight — "Huh, he just said these weren't the droids they were looking for, and they repeated it back in a mono-tone? Weird."
Arcana — "That stress pattern. The eye contact. His hand in his component pouch. That's a Suggestion spell!"
Perception — "Why is this guy keeping that glowing green crystal hidden while he talks?"
Players should pick one, whichever is most meaningful to their character.
The victim of Suggestion is unaware of the spell unless given cause to question it.
Points 1 and 2 above apply to external observers. The spell wouldn't be much good if the victim could detect it.
I would not typically give the victim a chance to notice the spell, unless they made their save.
Q&A
Why stealth?
It doesn't have to be stealth. You could make a good case for deception, or sleight of hand instead. I picked stealth as a "catch all," and a skill that many casters would benefit from anyway.
Why would whispering the verbal component of suggestion require me to make a stealth check?
It isn't clear whether or not you can whisper your suggestion. Or if you need to speak it loudly. Or like Obi-Wan Kenobi in Star Wars. Or how obvious using a material component is (does it glow? does it need to be manipulated in the hands? etc.?).
If you like, you can define all of these aspects of the spell. If Suggestion comes up a lot, this might add quite a bit to your campaign.
But most of the time, I find it's just easier to let the dice decide. Rather than having a prolonged debate about the exact motions required to cast suggestion, or where everyone is standing, or how easy it would have been to notice a Jedi mind trick in the wild, it's more expedient to make a skill check.
Even if they discerned I was up to something, it would already be too late to discover what before the effect took hold.
None of this is intended to be a way to stop Suggestion. The spell should do what it says it does, regardless of what anyone sees or notices. This answer is about who is aware of what happened after the fact.
Wouldn't it require somebody else to make an active perception check to see if they could hear it? On top of that, they would also have to make an insight check to determine if they even knew what I was doing.
In general, if a character can make an active insight/perception roll to notice something, they should be able to use their passive scores without an action. You could have them roll against a DC if you want to emphasize the use of active skill checks (and even have different DCs for passive and active checks).
I generally prefer opposed rolls (or rolling against passive values), simply because it gives the player a bit more control, agency, and time in the spotlight.
The most likely outcome is:
- Create and Destroy Water fails because the ocean is not an "open container".
If anything, the planet's crust is the ocean's container, but even that is a very permissive interpretation.
However, if the DM is permissive and lets the spell work then:
- You destroy 10 gallons of water, but the shark is not impacted in a significant way.
Remember, spells only do what they say they do: Create and Destroy Water doesn't state or imply that it is inherently dangerous or harmful, so it isn't.
Moreover, about the instantenous duration the PHB states:
Many spells are instantaneous. The spell harms, heals, creates, or alters a creature or an object in a way that can’t be dispelled, because its magic exists only for an instant.
Therefore, Create and Destroy Water's instantaneous duration is an indication that it can't be dispelled because the magic only exists for an instant. It is not an indication that the water is destroyed in 0.001 microseconds to create vacuums or other such effects.
Addendum:
As a D&D game term, "instantaneous" is just an abstraction. Create and Destroy Water can make rain fall, but how could rain literally fall instantaneously? Examples like these can be seen in most other instantaneous spells.
Best Answer
Yes
To play Devil's Advocate here, it is not strictly RAW to disallow spellcasting. Of course, it is a sensible house rule -- but it would be a house rule if you disallowed it.
Jeremy Crawford says you can
This tweet from Jeremy Crawford explicitly states that being underwater doesn't interfere with spellcasting. There is no conditional "Yes, if they can breathe underwater"
JC says you can, but only if you can breathe underwater?
Another tweet from Jeremy Crawford says that, if you can breathe underwater, you can perform the verbal components of spells. Fair enough. However, this is NOT the same as "if you can't breathe underwater, you can't perform the verbal components of spells" either.
Just as saying "if you can sing, you have a voice" is true, but "if you can't sing, you don't have a voice" is not necessarily true. Again, strictly speaking, nothing is disallowing spellcasting here yet.
The PHB says you have to be able to talk?
As @NautArch has shown, the PHB does mention a rule on V components of spells that seems like it should affect spellcasting.
This question on Quora asks if we can talk normally underwater. Well, the answer is yes, we can speak normally. The question is just, can the person you're speaking to understand you?
Well, in spellcasting, nobody needs to understand you. You just need to produce mystic words that form a combination of sounds, with a specific pitch and resonance. Note that you can always do this underwater, it's just that the sound is formed in your larynx and becomes distorted as soon as it touches the water. But the rules don't say "the sounds must reach outside your larynx" or "others must hear you clearly". You can technically still do it.
Moreover, every spellcaster will likely have different ways of casting the same spell, just because they naturally have different voices. It is not against the rules to consider that there are multiple ways you can set pitch and resonance, but still cause the weaves of magic to be set in motion in the same way. So, sound can still travel through water. Why can't a magic user speak those mystic words in a way that, when the sound travels underwater, the specific pitch and resonance still matches what is needed to pull off the spell? RAW, this is not illegal.
But Gagged prohibits spellcasting, so why doesn't being underwater?
There are many ways to wave this away. Any answer I give will not be RAW, and is in DM fiat territory absolutely.
Nonetheless, you can argue that when you are gagged, your tongue cannot move about and you cannot shape the sounds and words precisely because of this, whereas being underwater does not forbid this.
You can also say that being gagged restricts your jaw movement, but being underwater doesn't, so you still retain enough control to be able to cast while submerged.
Sensible House Rules
Casting underwater is different from casting in air, this is true. How you handle this is up to you. This Enworld discussion shows a few ways other DMs handle it, in the order of their appearance in that thread:
Another definitive JC Tweet:
You can cast a single spell underwater, but afterwards you begin drowning if you can't breathe underwater. Otherwise, no rule prevents verbal components from working underwater. Thanks to @mxyzplk for bringing this tweet to my attention!
Extra Note: In that Twitter thread, Dan Dillon asks:
To which Jeremy replies "yes." But technically it is Con mod rounds only, without the +1. The +1 only happens when you've held your breath and have Con mod + 1 minutes before you start drowning.