Fire deals damage to crew members touching it, but most importantly, drains oxygen from the room it's in, and it inflicts constant damages upon affected installations. It will also explode any of the Oxygen Tanks or Welding Fuel Tanks on contact (especially those that are in your inventory).
Fire is usually caused by electricity-related mishaps, most commonly overloading. This tends to result in multiple installations catching fires, overwhelming the crew. The thermal artifact will also set anything it touches on fire. Obviously, fire can occur during reactor meltdown.
Dealing with fires
Fires need access to oxygen to keep going, which informs the ways used to deal with them.
The Fire Extinguisher is the most obvious tool for the job, but tends to be insufficient when dealing with multiple fires at the same time.
It is possible to simply close off the doors to the room ablaze and let the fire use up all the air inside and die out. The process can be accelerated by cutting off the air supply of the room (see Oxygen Generator), but this is a risky and usually unnecessary endeavor. This is a viable, but slow way to take 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.
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 by another hazard, flooding, and for this reason the process 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 and makes sinking easier.
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 both cases, water will naturally flow toward the lowest available location not entirely flooded and begin to fill up rooms after rooms.
Dealing with flooding
A diving mask temporarily prevents its wearer from drowning, and a diving suit has the additional effect of protecting 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, to put it on quickly when needed. Without it (or access to it), swim toward breathable air hoping to reach it in time; until the flooding is dealt with (or you flee for a safer area) you will have to resurface regularly to take a breath.
Once immediate survival is ensured, the highest priority is to investigate and determine the origin point(s) of the flooding. In the case of a hull breach, the impact having caused the breach should be noticeable enough to orient the researches. The Status Monitor can help in this endeavour by mapping the flooded areas. Finally, one can follow the water falling downward or the current to locate the origin point of the flooding.
Doors can seal off sections of the submarine and prevent the passage of water; counter-intuitively, 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 areas; others point out that such a closed-off area will fill up quickly and crush the unfortunate trapped inside due to the rising pressure while opening doors would not allow the water to flow fast enough to have dangerous effects on the whole submarine before the problem is fixed. Use your wit and communication with your colleagues to determine the most appropriate course of action in your specific situation.
Pumps should be activated to empty out the 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 fighting the raise of the 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, sas should be closed, breach 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 by the color). 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.