Fire damages crew members touching it, inflicts constant damage upon affected installations, and—most importantly—consumes oxygen from the room it's in. It will also ignite any Oxygen Tanks or Welding Fuel Tanks on contact (especially those that are in your inventory), causing them to explode.
Fire is usually caused by electricity-related mishaps, most commonly overloading. This tends to result in multiple installations catching fire, overwhelming the crew. The thermal artifact will also set anything it touches on fire. Obviously, fire can occur during a reactor meltdown.
Dealing with fires
Fires need oxygen to keep burning, which informs the ways to deal with them.
The Fire Extinguisher is the most obvious tool for the job, but it tends to be insufficient when dealing with multiple fires at the same time.
It is possible to isolate and extinguish a fire by simply closing all doors to the room it's in and letting the fire use up all the oxygen. This is a viable but slow way to put out fire—useful when the affected installations can be ignored and maybe repaired afterwards, but insufficient when a fire needs to be put out quickly. The process can be accelerated by cutting off the air supply to the room (see Oxygen Generator), but this is risky and usually unnecessary.
Finally, desperate crew members might decide to put out a fire by drowning it. While this is an extremely efficient method to put out fire, it essentially replaces it with another hazard—flooding—and for this reason is usually not recommended. Note, however, that it may happen on its own during a regular, unplanned flooding.
Flooding is an omnipresent menace in Barotrauma. While items and installations are waterproof, the crew can easily drown; moreover, the intense pressure found in the ocean will crush unprotected crew members found in completely submerged areas, instantly killing them. On top of these immediately lethal consequences, flooding also weighs down the submarine, increasing the rate of descent.
The excess water can come from three sources: hull breaches, opened doors to the outside, and, a lot less likely, pumps being set to fill the submarine uncontrollably. In all cases, water will naturally flow toward the lowest available location not entirely flooded and begin to fill up room after room.
Dealing with flooding
A diving mask temporarily prevents its wearer from drowning, and a diving suit has the additional effect of protecting its wearer from pressure; putting on the appropriate equipment can save your life and give you the time necessary to take additional steps. Don't forget the required Oxygen Tank; cautious crew members frequently store this equipment in their inventory (taking care not to use up the oxygen beforehand) just in case, so they can put it on quickly when needed. If neither item is equipped or available, swimming toward breathable air is your next best course of action; until the flooding is dealt with (or you flee to a safer area), you will have to resurface regularly to refill your lungs.
Once immediate survival is ensured, your highest priority is investigating and determining the origin point(s) of the flooding. In the case of a hull breach, the impact that caused the breach should be noticeable enough to orient the crew and coordinate repairs. The Status Monitor can help, as it displays flooded areas. Finally, you can watch for falling water or follow a current to locate the origin point of the flooding.
Doors can seal off sections of the submarine and prevent the passage of water. Counterintuitively, this is a double-edged sword and a divisive topic for some players: Some advocate closing as many doors as possible to contain the flooding to a limited area. Others point out that such a closed-off area would fill up quickly, and the rising pressure would crush any unfortunate crew members trapped inside, whereas opening doors would allow the water to spread out, filling individual compartments more slowly and giving crew members additional time to repair the cause of the flooding—delaying dangerous effects on the whole submarine. Use your wit and communicate with your colleagues to determine the most appropriate course of action in your specific situation.
Pumps should be activated to empty out water as possible; it may be enough to undo the first cause of the flooding, but even when this is not the case, it will obviously help fight the rising water level.
Once the cause of the flood is located, it must be dealt with: malfunctioning pumps should be adjusted to pump the water out instead of in, the doors of the affected room(s) should be closed, and breaches should be fixed with a fueled Welding Tool. (If the exact location of a breach is uncertain, don't hesitate to wave the welder around on suspicious hull sections to check their status.) If you identify the cause and/or initial location of the flooding but are unable to fix it, use the chat to communicate this information to other crew members.