Wayne,

It's unlikely, IMO, that block heaters would hide any serious engine defect.
It can be advantageous to have the engine at 120 degrees before starting. A
quicker warm up means less time at high idle at the dock and is considerate of
your marina neighbors. For a Type A kind of person it can get you moving much
more quickly. Any diesel with a lot of hours will get a little fuel in the
crankcase while operating. Keeping the engines warm will help limit dilution
at start up.

I'd guess it is indicative of a maintenance concious owner to keep the block
heaters on while the engines are shut down. Especially if the engines appear
well maintained and the engine spaces are wiped down and clean.

Twin DD 12 bangers.... must be a sizeable boat or damm fast. :=)

Butch