So I was thinking about some options to run my players through that didn't involve simply ordering them to "Blow up target X." I was thinking of some other things that soldiers have to deal with, and came up with the idea of the character's platoon taking part in trying to put out a massive fire.
My idea is that they're dropped off on a planet that's had a battle going on for quite some time. So far the enemy xenos (minor unnamed, read "made up" species). Because of how well the battle was going, Segmentum Command ordered 80% of the ground troops into the next theater of war approximately a month before the players arrive, leaving the entire system being fought for by a small handful of well coordinated (but now hugely understaffed) force against an almost beaten enemy who's regrouping.
So far there's a single Sword class frigate in orbit assisting as possible. It's going to get ambushed by a pair of Eldar raiders and crash over the targets that it's playing fire support for (READ: The players).
It still has its thrusters going when it crash lands, but the commander wants to evacuate as much crew as possible, but I know of absolutely nothing in the 40k universe that would be effective at fighting fires. And this thing is going to be burning for a long time otherwise.
So far a friend of mine and I have discussed repurposing tanker vessels, and possibly using Marauders refitted to carry water (there's still a significant navy presence on the ground; Command didn't give them time to fully evacuate. Hooray orders). But as far as I know, there isn't anything specialized for dealing with a fire of that magnitude and capacity.
TLDR - How would you put out a major fire in the 40k universe?