I've owned two boats with diesels. The first had a Volvo MD2 diesel from the beginning and had no fan. Ambient air worked fine but also opened the lazarette hatch to let cooler air into the engine compartment. Ran this engine for more than 24 hours in the ITCZ when the temp was awfully warm without issues. Current boat had an Atomic 4 so came with a fan system. Took the plumbing and fan out because the deck exhaust fittings were bulky and in the way. Yanmar 3GM30 runs fine in reasonably warm conditions here in Hawaii. If I remember, open the lazarette hatch on this boat to get fresh air below but it doesn't seem to matter. If you have a fan system, couldn't hurt to run it. Exiting the exhaust into the cockpit might be enough to justify a fan if you sail in cooler climes.