Yep! Serious all the way! Engine compartment fire = Bad day at the lake! :banghead:
As an added safety measure, I open my engine compartment whenever I add fuel AND in the staging area at the boat ramp as I make final preparations to untrailer the boat. It is also recommended to run the fan for a minute or so prior to restarting the engine(s) any time you have had the engines off for a period of time, i.e. floating around on the water, or anchored/docked for more than say 5 minutes.
The engines are cooled by water being pumped through the cooling system, not by the fan. The fans only function is to evacuate fumes from the engine compartment.