A vessel engine room is close to an ideal total flooding enclosure. It is a steel compartment with defined and limited volume, positive boundaries, and controllable ventilation, which means a gaseous agent can reach and hold design concentration reliably. That is why fixed total flooding systems, historically CO2 and increasingly clean agent, are the standard approach in machinery spaces.
The hazard inside that box is concentrated. Diesel fuel lines, lubricating oil, hydraulic lines for steering and stabilizers, and fuel filters all run within a few feet of exhaust manifolds and turbochargers operating far above the autoignition temperature of everything around them. Bilge accumulation of fuel and oil adds a pool at the lowest point of the compartment. It is the same fuel-adjacent-to-hot-surface problem a generator room has, in a smaller enclosure, on a moving platform, with vibration accelerating every connection failure mode.
The sequence is what makes marine systems distinctive. Total flooding only works if the compartment holds the agent, and a running engine is drawing combustion air through the space continuously while ventilation fans exchange it. Discharging into a running engine room simply sends the agent overboard. Engine shutdown, ventilation shutdown, and damper and vent closure have to occur before or with discharge, and on many vessels a portion of that sequence depends on crew action rather than automatic function.