[RPG] How to legally trap a character under a rock, underwater

As noted by DoStuffZ, sound travels faster in water than air. The problem with underwater speaking is that we (human airbreather) need to exhale oxygen to speak, which forms bubbles and alter the sound. If you hum, you'll be heard quite well, so you can try to communicate this way.

I never heard of any official description of D&D / Pathfinder languages, so feel free to houserule that aquan is indeed a way of speaking clearly underwater even for non-amphibious creatures (by humming or very short syllables that wouldn't be altered too much by bubbling).

If you're looking about rules, note than D&D3.5 aquatic elves live underwater and do not speak aquan. Aquatic druid doesn't gain aquan either.

This link to Paizo forums also make a couple of points :

I found this in stormwrack I'm still looking, but this might shed some light. It's for a Bard: "Bardic Music: A bard performing underwater is restricted in what sorts of Perform checks she can make. Perform (sing) is impossible unless the bard can breathe water, whether naturally or by virtue of a spell. Oratory is similarly ineffective."

and

Spellcasting Underwater: Casting spells while submerged can be difficult for those who cannot breathe underwater. A creature that cannot breathe water must make a concentration check (DC 15 + spell level) to cast a spell underwater (this is in addition to the caster level check to successfully cast a fire spell underwater). Creatures that can breathe water are unaffected and can cast spells normally. Some spells might function differently underwater, subject to GM discretion.

This would means that speaking (and even casting spells with verbal component, though it says nothing about spell with language subtype) isn't a problem as long as you can breathe.

The Aquatic Adventures campaign setting addresses several new rules for water environments, shows tips to survive underwater and how things works in different situations that is not covered by the core rulebook.

Freedom of Movement

Here are some new rules and clarifications for Freedom of Movement in the book:

In addition to preventing a character from being grappled and several other nasty conditions, freedom of movement enables the target to move and attack normally while underwater. Thus, a creature benefitting from freedom of movement can swing weapons without penalties (though objects that leave its possession, like thrown weapons or ammo from ranged weapons, lose this benefit), lift objects without drag, and walk along the bottom at full speed. The creature automatically succeeds at all Swim checks as the water offers no significant resistance to its movement. If the creature has the ability to do so, it can fly or use spells like air walk to freely move around as if it were in air. Creatures who need just the movement benefits from freedom of movement without the benefits against grapple, paralysis, and magic effects that hinder movement should consider the lower-level spell free swim on page 59.

So, on top of avoiding the weapon penalties for being underwater, you can also ignore some of the underwater mechanics, like swim checks to remain in place and not be pushed away by the water currents.

Pressure

Most aquatic creatures are adapted to survive a certain pressure threshold, but non-aquatic creatures do not have such defenses, and thus, they take pressure damage. This is reflected as 1d6 damage per minute under 100 feet underwater, and an additional 1d6 for every 100 feet after that. The DC to avoid this damage is 15 plus 1 per minute after the first. Depending on the native range of each creature, sudden changes could also result in pressure damage.

For instance, if Valeros swims down 220 feet, he is 120 feet outside his native range of 0 to 100 feet, so he needs to attempt a Fortitude save every minute to avoid taking 2d6 points of damage. If he succeeds at five Fortitude saves, his native range shifts to 100–200 feet, and he must attempt Fortitude saves to avoid taking 1d6 points of damage each minute. After five more successful Fortitude saves, his native range is now 200–300 feet, and he takes no further damage from pressure, but if he ascends back to the surface, he must resume attempting Fortitude saves to avoid taking 2d6 points of damage from the reversing pressure.

(...)

Spells like freedom of movement and life bubble prevent pressure damage. Pressure adaptation and stabilize pressure on page 61 are more focused and lower-level options.

Freedom of Movement avoids this damage completely.

Currents and running water

Water sometimes is not static, its moving and characters swimming are being moved by water currents. Oceans and rivers work similarly in this matter, meaning that characters have to always make a Swim check with a DC based on the current strength, as described on the Swim skill. With a caveat, oceans are always at least rough water (DC 15), and faster ocean currents count at least as stormy water (DC 20).

Freedom of Movement removes the necessity of this check.

Off-balance and prone underwater

Yes, the core rulebook says that neither flying nor swimming creatures can drop prone. However, aquatic and swimming creatures can go off-balance when underwater, which works similarly to the prone condition: opponents gain +2 to all attacks against it and all attacks it makes take a -2 penalty using piercing weapons and cause half damage.

But how a creature becomes off-balance? By failing Swim checks. A trip attempt against a creature underwater forces them to make a Swim check with a DC equal to the result of the maneuver, and if they fail, they go off-balance. Many aquatic creatures can't be tripped, and can take 10 on swim checks, so this is hardly a problem to most of them. They can return their balance by making another swim check on their turn.

Freedom of Movement allows you to automatically keep your balance.

Buoyancy

This is the last, I promise. Basically buyoancy makes a creature go up or down when swimming. Whenever they fail a DC check by 5 or more, or when they do not spend any actions to swim, the creature either rises or sinks due to buoyancy. It has three normal states: sinking, rising and neutral.

Land creatures usually have a sinking buoyancy, unless they are carrying a substantial amount of low-density gear (like many wooden items) and little or no high-density gear (items made of metal or stone). The book presents air tanks and gear that can change your buoyancy on demand.

Buoyancy also has two extreme states: swiftly sinking and swiftly raising. If they are swiftly sinking, their carried gear is so dense that they can walk at the bottom of the body of water, and must make a DC 20 check to stay off the bottom. If they are swiftly raising, they must make a DC 20 Swim check to stay submerged, or they will move to the surface of the body of water.

The speed of this buoyancy-based movement depends on your Swim checks. Whenever you fail (or do not make) a Swim check, you move up or down 10 feet, and every turn after the first that you do not attempt to fix that, the speed increases by another 10 feet to a maximum of 30 feet per round after 3 rounds. When swiftly sinking or swiftly rising, a failure means you immediately move 30 feet in that direction. Also, while moving like this, the creature is considered off-balance.

When moving against your buoyancy, your movement is hindered, much like a flying creature flying upwards. As such, each 5 feet of movement in the opposite direction of your buoyancy costs you 10 feet of your movement.

Freedom of Movement allows you to automatically keep your buoyancy controlled.