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Are there any special rules for invisibility when underwater?

For example, Pathfinder (a different game) has a rule that makes it easier to detect invisible creatures underwater due to the way they displace water (I haven't quoted the text in order to avoid confusion).

Are there any similar rules in D&D 5e?

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There is no specific rule

The Basic rules impose limited restraints on a creature that's underwater: Movement speed is halved for creatures without a swim speed, most melee weapon attacks also have disadvantage for those creatures, and ranged weapon attacks are limited to the 'normal range' of the weapon (i.e. the first number of the two given for weapon range). (Basic rules, p.8)

In fact, the only other rules I know of with any bearing upon hiding underwater are on page 117 of the DMG, under the heading Underwater Visibility. These rules explain that creatures who are underwater will only be able to perceive enemies who are less than a certain distance away, depending on visibility conditions.

There is a general rule: Imposing disadvantage

5e is light on specific rules for particular cases, and prefers to equip DMs to make rulings in response to the actions of their players. One of the best tools provided for this is the ability to impose disadvantage when 'circumstances hinder success in some way' (DMG p.239).

If a character is attempting to use their invisibility to hide whilst underwater, you could impose disadvantage of their stealth check to reflect their difficulty in so doing*. If spotted, they would still be unseen, but they would no longer be hidden - the main difference is that an enemy would know enough about their approximate location to target them with a disadvantaged attack.

* I'm assuming here that it would be more difficult to remain undetected underwater, since that's the assumption in the question. I have no idea if it's true or not.

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    \$\begingroup\$ I think this answer recommends a good general tool, but I do wonder whether the tool is appropriate to the situation. Since seeing underwater is in general more difficult, it seems to me that the "moves water" somewhat washes out. After all, an invisible character "moves air", and there's no Disadvantage for that. The Disadvantage would be -- in either case -- if the dirty air or water makes it obvious that something is moving, which cannot be attributed to current. \$\endgroup\$ Oct 8 at 13:21
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    \$\begingroup\$ Invisibility is actually plausible underwater—clear sea creatures whose bodies have the same index of refraction as water actually exist. They really can’t be seen, their presence or absence has no effect on light passing through so there is nothing for eyes to detect. This kind of rule only makes sense at the surface, where ripples and splashes would be visible. \$\endgroup\$
    – KRyan
    Oct 8 at 13:45
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    \$\begingroup\$ @KRyan that's an interesting point. I think I imagine an invisible creature as a bubble of air. But why not as a bubble of water? I guess magical invisibility is impossible to simulate. \$\endgroup\$
    – Lovell
    Oct 8 at 16:24
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Frame Challenge

Why should there be such a rule? In reality, an object is invisible if it's transparent and has the same index of refraction as the medium it's in. For example, there are polycarbonate plastics that have the same index of refraction as water; they are effectively invisible when placed in water.

If invisibility works by magically making the character transparent and altering their index of refraction, then since the human body is an ugly bag of mostly water, you'd expect that invisibility would work better underwater.

On the other hand, if invisibility works by magically replacing the physical substance of the character with the medium they're surrounded by, then invisibility would still work just as well underwater. Strange things would happen if it were cast while the character is in air and they then moved into water... or the opposite!

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    \$\begingroup\$ We don’t even need to get into polycarbonate plastics—there are real, living sea creatures with that property, too. Might not be as perfect a match as the plastics, but it’s certainly good enough—and magic can fill in for any imperfections in real biology. As for your last paragraph, though, you assume it doesn’t update as the creature moves into new mediums—which isn’t a given, either! Anyway, since invisibility is an illusion spell, not a transmutation, presumably it’s manipulating light, not substance. \$\endgroup\$
    – KRyan
    Oct 8 at 18:28
  • \$\begingroup\$ I could see disadvantage on stealth for an invisible creature at or near the surface of the water -- if you're swimming or even just moving around much, you're going to generate visible ripples. It's basically the same as walking through thick dust or scattered flour while invisible, you can't be seen but you're leaving a lot of clues about your location. \$\endgroup\$ Oct 11 at 13:36

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