I'm guessing this was implemented post-Conception (I observed it a month ago on a trip):
Nautilus Explorer has, directly over the camera table on the dive deck, two large-ish capacity extinguishers mounted upside down with thermal triggered sprinkler heads inserted into them (upon quick glance was similar to star type commonly seen in buildings).
I did not look into the details of extinguisher type, etc. Presumably they put some thought into it.
Not directly supervised charging only permitted on deck on a mostly metal table and directly below an automated suppression system.
Seemed a very simple, relatively cheap and logical solution to the eyes of a layman. (and easy to comply with)
I'm sure there's a fire sprinkler desinger/engineer on here who could comment more specifically on potential effectiveness?
I think there was a hose hookup for supplemental water on to the area as well. And the deck fire hose was 10' away.
It might not be as hard/expensive as the discussion above is pondering?
Someone has, at least, taken a shot at it in the real world.