…as a big simplification in naval war games, not as a real-life military rating for a ships durability under actual fire.

Technically that was "life". It's just that conceptually it was hitpoints.

In chainmail, the war game dnd is historically based on, a hit point is a fraction of a hit dice, each HD represents one soldier, roughly, so 12hp being the average of 4hd isn't too bad, no.