The only reason you'd have blowers with diesel engines is to remove excess heat from the engine compartment, if you installed a large case alternator, for example. Diesel doesn't produce dangerous fumes like gasoline does.
Not sure how it applys to the cat your looking at. It is nice to dissipate the heat out of the boat after motoring during the warmer days. mine vents in below the cockpit coaming and makes a nice hand warmer during the colder months.
If the space is well ventillated absent blowers, they would be an unnecessary complication. If not, however, they will improve the performance of your engine(s) and help remove heat from the boat if you have an exhaust to the exterior of the yacht. We have a blower that pumps air into the engine space near the bottom of the compartment and passive ductwork to the top that discharges through the transom. The stream of heated air out of the exhaust ducting is quite impressive, particularly during the summer months.
__________________ "It is not so much for its beauty that the Sea makes a claim upon men's hearts, as for that subtle something, that quality of air, that emanation from the waves, that so wonderfully renews a weary spirit."