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Underwater combat is never fun but for magic users it can be especially frustrating. Unless a character has Water Breathing or uses Alter Self they may find themselves in a situation where the DM rules that they cannot cast any spells with Verbal components--i.e. most of them.

At a gaming session my friends and I debated how effective most combat cantrips would be in an underwater environment.

We figure that most distance attack cantrips would be rendered useless: Acid Splash and Poison Spray would be diluted upon casting, Ray of Frost would turn the water immediately in front of the caster's fingers to a ball of ice, Fire Bolt and Produce Flame would be quenched immediately upon casting, etc.

But we weren't sure about a few Cantrips. Eldritch Blast would likely work as normal, but what about Sacred Flame which uses radiant energy? Would Shocking Grasp electrocute the caster as well as the target?

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A spell only does what it says in the description

None of the spells you list, nor any spell you can cast, would be affected by being underwater except for the resistance creatures acquire against fire.

Creatures and Objects that are fully immersed in water have resistance to fire damage.

You can narrate this by justifying some magical force holding the energies of the spell together warding against water until the spell ends.

The full list of underwater combat rules can be found in the Player's Handbook (p. 198) which includes modifiers to weapon attacks but nothing regarding spells.

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    \$\begingroup\$ Note that there is some debate about the use of verbal components underwater, but this affects the casting (not the effects) of the spells. \$\endgroup\$ – Rubiksmoose Mar 18 '18 at 19:59
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    \$\begingroup\$ I think the only limitation on verbal components underwater is that if you start speaking or chanting underwater, you can no longer hold your breath and thus start suffocating; you'd survive a number of rounds equal to your Constitution mod, and then drop to 0 HP. \$\endgroup\$ – V2Blast Mar 18 '18 at 20:39
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    \$\begingroup\$ @V2Blast Read the Spellcasting description on verbal components. The debate revolves around whether the sounds constitute the components or the activity of your vocal chords. (JC has said that whispering doesn't count for example) \$\endgroup\$ – David Coffron Mar 18 '18 at 20:52
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    \$\begingroup\$ Ah, okay. I mean, you can still make sounds underwater... it's just that the air expelled as you make those sounds will be replaced with water filling your lungs :P \$\endgroup\$ – V2Blast Mar 18 '18 at 20:57
  • \$\begingroup\$ @V2Blast only if you breathe in afterward. I can make sounds under water with part of my held breath and continue holding the rest; though a DM may simplify the implementation to avoid tracking oxygen gauges for each character. \$\endgroup\$ – Asher Mar 19 '18 at 12:14
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These are all of the underwater rules I could find. In addition, there are tweets by JC and Sage Advice where it is definitely allowed to cast spells with Verbal components if you can breathe (and maybe allowed even if you can't, but you probably start drowning). The rules are clearly deficient in this area.

The underwater combat rules only apply to weapons, but there may be some parallels.

Underwater Combat (PHB 198)

  • Melee
    • Creatures with swim speeds are not disadvantaged
    • Attacks with daggers, javelins, shortswords, spears and tridents are not disadvantaged
    • All other attacks have disadvantage
  • Ranged
    • All ranged weapons automatically miss beyond their normal range
    • Attacks with crossbows, nets (adjacent) and weapons thrown like javelins (e.g., spear, trident, dart) have no disadvantage at normal range
    • All other attacks have disadvantage

This is the closest we have to rules about how far you can see underwater. At least we see the recognition that you can't see as far away underwater. There shouldn't be any restrictions based on sight that don't also apply to ranged weapon attacks.

Underwater Visibility & Encounter Distance (DMG 116)

  • Clear Water, bright light - 60 ft
  • Clear Water, dim light - 30 ft
  • Murky water or no light - 10 ft

Water is never defined as an obstruction w.r.t. area of effect spells, but if the DM considers it to be one it should affect all area spells.

A Clear Path to the Target (PHB 204)

If an obstruction is between you and the targeted area, the spell takes effect at the obstruction.

W.r.t. Acid Spash & Ray of Frost, the spell descriptions do not give any support to the idea that they would be diluted during their journey from caster to target, so I don't see any reason to impose that on them. These are magical attacks after all. Even poison spray need not be diminished without a rule saying it should be.

This should ultimately come down to the DM, published adventures (if they are underwater, maybe they will define the rules better), or rule updates. I recommend that you come up with universal rules based on how far/well you can see underwater and refrain from nerfing spells based on their narrative descriptions in the absence of actual rules. It would be one thing to reduce the range of all spells underwater by half across the board, but another to pick and choose how far each spell can work underwater by the fluff in their descriptions. You might also consider imposing disadvantage for spell attacks with somatic components if the caster doesn't have a swim speed. Possibly spells with material components are not possible, but using a spellcasting focus imposes no disadvantage. This all falls under house rules/DM adjudication which you should be able to come to terms with prior to ever running an encounter where the rules are needed.

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    \$\begingroup\$ +1 for this important point: refrain from nerfing spells based on their narrative descriptions in the absence of actual rules The urge to simulationism is strong in many of us playing a variety of RPG's. \$\endgroup\$ – KorvinStarmast Mar 19 '18 at 19:09

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