Where is the boat being stored? Sounds like it's indoors? My 1999 Merc
5.7 has a similar setup. One drain on each manifold, two on the block,
and one on the circ. pump hose at the low point. Has the engine been
winterized? Draining the water will prevent freezing damage to the
block but just leaving it empty is inviting corrosion in the coolant
passages.

When I winterize (gets pretty damn cold here in CT) I'll run the engine
on the hose until it's up to temp (175F) and the thermostat opens -
then shut it off and pull all the drain plugs. After it's drained the
plugs go back in and I restart the engine while my wife pours marine/RV
antifreeze into a funnel going to the earmuff on the drive leg. It
takes over two gallons to fill the entire engine, and once the exhaust
starts spewing red liquid I fog the carburator with preservative until
the engine quits and I'm done. Last step is have the boat shrink
wrapped as it sits outside all winter.

Being a RWC engine I doubt there's enough residual water left in your
engine after pulling the drains to do any damage if it froze, but for
$10 in antifreeze why take the chance? And then there's the corrosion
issue.