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The PHB says normally you need "a clear path to the target" to target someone with a spell (see "Spellcasting", page 204). Jeremy Crawford confirmed, that you can't cast a spell through semi transparent things.

Water is quite "semi-transparent" for me. Can I normally cast a spell through water? For clarity, let's say the spell is Guiding bolt, I can see the target, and the situation is one of the following:

  1. There is a waterfall between me and my target
  2. My target is in a pond, beneath the surface, and I am above it
  3. Both me and my target are underwater
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    \$\begingroup\$ That linked answer states that objects block spells regardless of transparency, not because of it. Air is semi-transparent. \$\endgroup\$ – Derek Stucki Oct 2 '17 at 14:05
  • \$\begingroup\$ @DerekStucki the question remains the same though - does water block spells? \$\endgroup\$ – enkryptor Oct 5 '17 at 15:35
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The answer you linked has also quoted Mearls tweeting that:

in general, a barrier that stops physical objects stops spells

Based on this I would say that water does not stop objects and thus would not stop spells either. Of note is that it also does not provide any cover (no such thing is mentioned in the section for underwater combat, PHB p.198).

You have to keep in mind though that forming the verbal component of a spell might be challenging if you cannot breathe underwater.

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  • \$\begingroup\$ Comments are not for extended discussion; this conversation has been moved to chat. \$\endgroup\$ – mxyzplk Oct 2 '17 at 23:32
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The rules don't state that water, by itself, blocks spells

I would expect that if this was the general rule, or something specific to underwater combat, it would have been so stated in the rules. If you can see the target you meet the condition to cast the spell. Cover (see the rules on cover in the PHB) may or may not apply to a given situation. You'd need total cover to be prevented from casting the spell.

Cover (Summarized, SRD p. 96)

Walls, trees, creatures, and other obstacles can provide cover during combat, making a target more difficult to harm.
A target with half cover has a +2 bonus to AC and Dexterity saving throws.
A target with three-­quarters cover has a +5 bonus to AC and Dexterity saving throws.
A target with total cover can’t be targeted directly by an attack or a spell, although some spells can reach such a target by including it in an area of effect. A target has total cover if it is completely concealed by an obstacle.

Underwater Combat (Basic Rules p. 77)

Underwater the following rules apply. When making a melee weapon attack, a creature that doesn’t have a swimming speed (either natural or granted by magic) has disadvantage on the attack roll unless the weapon is a dagger, javelin, shortsword, spear, or trident. A ranged weapon attack automatically misses a target beyond the weapon’s normal range. Even against a target within normal range, the attack roll has disadvantage unless the weapon is a crossbow, a net, or a weapon that is thrown like a javelin (including a spear, trident, or dart).
Creatures and objects that are fully immersed in water have resistance to fire damage.

Those are the specific rules to various limitations in underwater combat. Each situation where one casts a spell underwater will need to be considered depending upon whether or not the somatic, verbal, and physical components can be applied.

Let's get down to cases

  1. In the case of guiding bolt that you ask about, unless the conditions prevent Verbal or Somatic components from being used, the spell will take effect if the caster can see the target. Whether or not 1/2 or 3/4 cover is provided would influence the attack roll's success, but does not prevent casting the spell at that target. Total cover would tend to get in the way of "that you can see" as a condition for using the spell for an attack. (Such as Guiding Bolt).

  2. The pond case is unclear. (Pun intended). I would rule that the spell can be cast, since the caster can see the target. An good argument can be made that partial cover (1/2 or 3/4) would be applied, but I find that more germane to arrow or spear (physical) attacks than spell attacks... because magic. The pond isn't "an object" but is a fluid. (Air is also fluid ... but now we are getting into simulation, and that's beyond the scope of the rules as presented).

  3. The underwater case is very clear. You can seen the target. Unless the conditions prevent Verbal or Somatic components from being used, the spell will take effect if the caster can see the target.

I'd expect a caster to use a water breathing or similar spell (or item) to engage in underwater combat. For someone lacking that, the issue of whether or not the verbal component works looks like a situational ruling. 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." That said, the official Sage Advice rulings compendium doesn't mention underwater combat.

Water Breathing(SRD p. 191 spell description)
This spell grants up to ten willing creatures you can see within range the ability to breathe underwater until the spell ends. Affected creatures also retain their normal mode of respiration.

FWIW, @Slagmoth makes an interesting point about underwater spell casting in this comment:

The linked answer {in the question} would indicate Sahuagin and Marids not being able to cast spells in their native environment though, which I am certain is not intended.

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  • \$\begingroup\$ Comments are not for extended discussion; this conversation has been moved to chat. \$\endgroup\$ – mxyzplk Oct 2 '17 at 23:32
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Only solid or fully opaque mediums can block spellcasting.

One ballpoint method of knowing if spellcasting can be done is:

  1. If you don't have to guess the target's square, and
  2. a ranged attack with a weapon could hit the target, then you can cast your spell.

The above applies to:
* The spell's range is not 0, self or touch.
* The spell description states a target "that you can see"

It doesn't matter if the attack would be made at disadvantage or the range of the weapon was impaired.

One thing a DM can rule is to greatly reduce visibility ranges underwater, to emulate murkyness and the water effects on light.

Credits: http://wildcatlighting.com/Light_Chart.html (Water penetration in water) For a crystal clear deep ocean, sunlight only penetrates 200m/650ft. On coastal waters, it is up to 50m/160ft. The first half of this range is where humans can see without penalty, the remaining range is low-light. Beyond that is pitch darkness.

I won't attempt to state how much light sources should reduce by. It depends on each DM's ruling.

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  • \$\begingroup\$ For those who like the "real world physics in the D&D world" feel, this illustration is IMO helpful for a DM to use as a reference. \$\endgroup\$ – KorvinStarmast Oct 2 '17 at 23:32

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