Based on your apparent background, which I take is Navy, your question presumably has to do with larger ships (+300 LOA).
My comments below probably won’t pertain to you, but here goes:
For vessels smaller than 300 feet LOA “A” is one answer, but on smaller vessels you may not find isolation valves in the fireman.
Some vessels will have EDFP (emergency diesel fire pumps). These are portable, and draw water from the sea. At the company I work at, during annual ff training, setting up the EDFP early on in a fire, even if not needed it, is SOP, so that if fire main pressure suddenly drops, you’re set.
My personal observation when it comes to the free-surface/weight of water issue is: a) yes, it can be a problem, and b) don’t worry about it in an initial attack. For belowdecks ff, using water to extinguish the fire is initially a matter of generating steam to reduce the O2 level in the compartment to <15%, to knock down the flames. This takes comparatively little water. Then, solid water is used to break Class-A ember materials up, but this should be a targeted matter, not spraying water willy-nilly.
You mention 2000 GPM. Using that flow continuously for 30 minutes means introducing 258 short tons of water into the ship. But a half hour of fire fighting time is a lot of time. 258 tons, even if introduced into the upper decks of a comparatively small ship may not compromise stability, and on the weather decks, of course, means nothing at all.
Water application in compartments is an arcane subject. Go to different marine ff schools and you may get different answers.From listening to experts, and by running a realistic ff simulator, I suggest this: whenever possible in a direct attack on a big, hot fire down below, put a bulkhead between you and the fire. Shoot the water through the doorway into the overhead. Rapidly target several spots in that overhead. Then close the door quick. If things go right, the water will flash to steam, expand and reduce the O2 level to below 15%. Flames out. (Caution: if you are foolish and leave the door open, the steam will reach out and scald you.) Wait a few minutes. Then repeat.
With vessels with large amount of Class-A materials aboard there is more to it than that.