Sorry to be catching this so late. Didn't even know we had a boat section. I wrenched on boats for 8 years before current job. All the water gravity drains out of the sterndrive (big outside part) . Engine holds a 2 to 3 gallons. When I "winterize" a boat like that, I have a special rig to run RV antifreeze in through the water pickup, and it takes about 2 gallons for the discharge to get "pink" like the antifreeze.
30 degrees over night...sleep well and don't worry. High of 25 degrees, concerned. High of 25 for 3 days, goner. To drain water you should have a minimum of 4 blue plugs, depending on year model (blue because it is a mercruiser, OMC and volvo use brass). Considerably older Mercruisers may have brass plugs or petcocks. One on bottom side of each exhaust manifold, and one on each side of block. If newer, you may also have one (or more) on the thermostat housing and/or one on the underside of the "y" where upper and lower water hose merge to water pump hose. pull those and let it sit. All the water will drain. For low of 30 degrees, throw a $5 clamp light in there and have a good night. Hope this helps for future reference.
Tip, trim the sterndrive down. Do not leave it up. Water can accumulate in the nose cone and crack the foot. Rare but it happens.
