Part of my training for doing "engine room checks" on a boat involved checking for any unusual smells, e.g. fuel leak, burning oil (from generator/engine), burning coolant (from generator/engine), or burning rubber (from sea chest raw water impeller). All of the components in there are equipped with sensors[1] that measure levels, temperature, etc. Perhaps there is room for a new olfactory sensor there? Aside from avoiding catostrophic issues like fire and engine or generator failure, it's also important to not pump out[2] any water from the compartment into the ocean if it's contaminated with oil, fuel, or coolant (the laws about this are super strict).

[1] There are digital sensors that are readable directly from the pilothouse by the captain which are rigged to automated alarms, as well as manual sensors (e.g. a pressure dial) that are readable from the engine room itself, for redundancy. So I don't think an olfactory sensor would replace the unusual smell check, but it could maybe augment it.

[2] The "bilge pump" is used to pump out water from the bilge (bottom floor cavity of engine room). To be honest on my vessel the policy is to never turn on the bilge pumps in the engine room at all because the risk of dumping contaminants is too high. But I still thought to mention this just in case there's an idea there.