1. If your DCC (Damage Control Capacity) drops lower than the damage levels (flooding and/or fire) your ships is doomed.
2. DCC seems to be defined by shipsize(tons)/crewsize. While the first is a constant factor, Crewsize will obviously drop during battles due to casualties reducing your DCC.
3. Armour is a very important factor. Thick armour means that Damage will stack only slowly and the crew has more time fixing Damage levels. German BB/BC will have an edge here because of their better armour and due to brittle Brit AP Shells.
Unarmoured ships are very vulnerable and have to be pulled out of battle early. Give the crews time to fix the ship if you don´t want to lose it.
4. Note the different DCC´ characteristics of specific shipclasses/types:

Battleships/Armoured Cruisers:
DCC usually something around 70-90%.
Notes: Good overall DCC due to being "true" warships. Large crews corresponding to their size. Starting crew is usually well protected so drop of DCC due to crewloss isn´t much of a factor for them. This means their DCC deminishes little during combat

Light Cruisers DCC usually something around 70-80%.
Notes: Generally the same for BB/AC holds true for Light Cruisers except that they are less protected which means the crews are more exposed. Thus their DCC drop during combat is more of a factor to watch out for compared to BB/AC

AMC/XC/ML: DCC usually something around 50%.
Notes: Large, unprotected ships with rather big crewsize. Still they aren´t true warships and having no armour means damage will quickly stack. Those ships will repair damage very slowly during combat and can take little pounding. They also are quick to catch fire and Flooding will quickly accumulate on both sides (due to shells penetrating one side and exiting the other). Watch out for Firelevels raising above 15% and Floodlevels above 20%.

Monitors: Can´t say much about them due to lack of expierence with them but i´d guess that the same rules of thumb for AMC/XC/ML are in place for them as well, except for the fact that some of them are better armoured.

DD/TB: DCC usually 80%-110%.
Notes: Small sleek hulls with comperatively large crews means they have high DC starting levels. Yet crewsize is here the deciding factor due to the crews being highly exposed. Crew losses can quickly raise to fatal levels where DC will drop so much that you can´t even fix minor damages. I have seen ships with <5% Flooding damage being eventually lost at the end of the battle because the surviving 5 man crew couldn´t do anything about it. Be really careful with the Division leadships, watch for crewlevels dropping down to less than 50% and pull them out. While they can usually brush aside small calibre Pom-Poms, everything above 65mm will hurt the crew (alot).
I think this is (way) overrepresented now and usually lose my DD/TB not due to lots of damage but due this excessive loss of crew that impares DC

1. "Uncontrolled Fires" indicate a certain loss
2. Fires are the biggest enemy of unarmoured ships, they have a hard time fixing fire levels and crew losses will quickly accumulate
3. For armoured ships anything above 50% is critical, above 30% dangerous, divide the figures by 2 when it comes to less protected/unarmoured ships
4. Pulling out, seizing fire and slowing down (even stopping) really works wonders here due to the wind not being able to feed the fire
5. Albeit not as bad as in DG, Fire still "eats" your crew away while it rages so your Crewsize will continously drop. Thus DC should surpass the fire levels.

Rule of Thumb:

Fire Level x 1.5
---------- > 1 = Ship is probably a loss

1. 100% Persistent Flooding on a side indicates a certain loss. Did i mention Torpedoes yet?
2. As long as the DCC is still high on a ship crews can fix even bad flooding, as long as DC is bigger than accumulated Flood levels.
3. Whether a ship is armoured or not doesn´t mean that much when it comes to flooding
4. Pulling a ship out, slowing down and stop firing works here as well, except that slowing down below 2/3 of the ships current capable speed doesn´t seem to have an impact. The pumps have to be running and stopping at least doesn´t seem to worsen the situation.
5. Propulsion damage is another factor to watch out, without propulsion no working pumps thus flooding quickly accumlates. I think propulsion damage can be repaired rather quickly during a battle
6. As soon as your Flood damage exceeds your DC you will get a red "Flooding exceeds Pumping" message in the ships information screen, indicating an almost certain loss. So the rule of thumb isn´t that vague at all:

Rule of Thumb:

Current Flood Damage
------------------------- > 1 = Ship is almost a certain loss

!!! Note that Fire and Flood damage count together vs. your DCC so i´d advise this last rule. Act before things get this bad!

Current Flood Damage + Fire Level x 1,5
-------------------------------------------- > 1 = Ship is a certain loss