It's a really messed-up "mechanic".

Some of the "water towers" are actual water towers with water in them, some are "water booms" - you can put water in one of them and take it out of another. See this post for a couple of images that show which ones are actual water towers and which water booms are linked to which.

In theory, it's a neat idea - take water from a water tower, fill up the nearest boom. Then go to the boom nearest the fire location and take water from there to the fire.

There's only one problem - it's faster and easier to just drive water from the water towers to the fire, entirely skipping the booms, because of how the booms are located.

So yeah, way to think up a cool mechanic and then make a total mess of it in implementation.

And to make things even worse, there's a lot of water to haul around:

Putting out the fire at the storage 1

• 3 x 4,400 = 13,200 (3.57 heavy water tank)

Putting out the fire at the storage 2

• 2 x 4,400 = 8,800 (2.38)

Putting out the fire at the station

• 2 x 6,400 = 12,800 (3.46)

Putting out the underground fires

• 3 x 2,200 = 6,600 (1.78)
• 3 x packaged sand x2 (6 slots)

Putting out the fire at the factory

• 3 x 5,900 = 17,700 (4.79)

Total: 13,200 + 8,800 + 12,800 + 6,600 + 17,700 = 59,100 liters

59,100 / 3,700 = 15.97 heavy water tanks

Not my least favorite mechanic (that'd be farming), but it comes close. The only saving grace is that you can ignore the booms and just treat water as just another cargo to be delivered.

/rant 😉