Blower is required, by certain rules
This spring I got a 2004 Catalina 36 and had it surveyed. The surveyor was checking to see if the blower was working and I stated my surprise that it even existed on a diesel. I have never noticed one before and have never used them. He said that it was a requirement by certain yacht building rules so the later model boats included them, but nobody ever used them because diesel boats don't have a habit of blowing up like the old gas ones did.
Engine compartment blower switch is a discrete little toggle switch on the bottom of the engine control pod. I've chartered similar boats in the past and never noticed it.