I wrap rope caulk (Mortite caulking chord from ACE Hardware) around a wooden bung and insert into the drain hole in the 1st stringer. The stops mast water from flowing into the entire bilge during winter. It's easier to manually remove the water (hand bilge pump and sponge) when its confined to just one bilge section. It also prevents damage to the bilge pump, float switch and AC raw water strainer should it flow into the rest of the bilge and freeze. I add anti freeze only to the forward bilge section. Thought I might share this with others up here in the northern climates.
Paul & Wendy Keyser
2005 C36 MKII #2257
Retired C36/375IA Mk II Technical Editor