🌊Water

Water has a lot to talk about, so I'll break it up into 4 sections: Aquatic combat, underwater combat, water pressure and drowning.

Aquatic Combat - This refers to combat that happens in and around water, but typically not completely submerged. As far as visibility is concerned, most of the time you can see to the bottom of lakes and rivers unless they are particularly murky. Oceans, seas and particularly wide lakes or rivers are the exception to this, where they can be deep enough that even in ideal conditions you can't see all the way down.

When a creature not in water attacks another creature that is at least waist deep in water, they have cover. Creatures not in water cannot attack creatures that are completely submerged if they are more than 5 feet deep.

Underwater Combat - Fighting underwater can make many fighting styles completely useless. Projectile weapons like guns can't fire at all. Thrown weapons don't function at all either. Bows and such have their range reduced to 10/20 ft. Melee combatants take a -4 penalty to attack unless they use a piercing weapon.

Many of these things have workarounds. Certain types of weapons can be specifically designed for underwater use. Aquatic creatures, amphibious creatures and creatures trained in athletics figure out methods to ignore the melee attacking penalty.

Water Pressure - Roughly every 30 ft you descend into water, you double the amount of pressure your body experiences. Going past 150 ft is usually impossible without being an aquatic creature, special equipment or magic. If you do not have such aid, then going deeper than 60 ft is usually a bad idea. In combat you don't have the luxury of slowly ascending to avoid the bends.

If you ascend more than 5 feet a round after diving to this depth, you must make fortitude save or become drained 1 and dazed. The DC is 20+every additional 5 feet you moved (10 feet DC 21, 20 ft DC 23 etc.). Even if you pass your save, you are still drained 1.

Drowning - Most creatures can pretty handily hold their breath for 30 seconds while under duress (or 5 rounds). For every point of strength you can hold it for an additional round. Every round past this limit inflicts drain. The first round you drain 1, the second drain 2 etc, causing you to rapidly die if you are unable to somehow breathe.

With specific equipment, magic or training, or being aquatic/amphibious, you can increase or eliminate this limit entirely. The most common way is gillweed.