The stakes

A raw-water-cooled MerCruiser holds water in the block, manifolds, and drive after every run. One hard freeze with water in the block cracks it — the most expensive preventable failure in this trade. Never assume drains are clear; verify each one flows.

Step-by-step

1. Run the engine to temperature (muffs or on the water) with stabilized fuel so the thermostat opens and treated fuel fills the system. Change engine oil and filter while warm — never store on used oil.
2. Drain the raw water system. Locate every drain point: block drains (both sides on V6/V8), exhaust manifold drains, power steering cooler, and hoses that trap low spots. Check the service manual for your engine's full drain map — late models have single-point or air-actuated drain systems.
3. Probe every drain hole with wire — rust flakes and sand plug them, and a plugged drain means water stayed in that jacket. If nothing comes out, do not move on; find out why.
4. Pull hoses at low points (raw water pump inlet, cooler lines) and drain them.
5. Optional but recommended: run non-toxic antifreeze through the system after draining (drawing it in with the engine running briefly, or backfilling drains) — belt and suspenders, plus corrosion protection. Antifreeze without draining first just dilutes into water and can still freeze.
6. Fog the engine per the service manual's storage procedure.
7. The drive: drain and refill gear lube (milky = pressure test before spring), grease u-joints/gimbal via fittings where present, inspect bellows, and store the drive down so the bellows aren't stretched and water can't pool in the exhaust.
8. Battery out and on a maintainer; block off exhaust outlets against mice; note everything done on the invoice.

Common mistakes

• Trusting a drain that dribbled for two seconds — probe it
• Forgetting the power steering cooler (a very common freeze casualty)
• Storing the drive trimmed up outdoors
• Winterizing the engine but leaving water in a heater, ballast, or washdown circuit

When to walk away

If drains produce nothing on a badly rusted engine and you can't confirm the jackets are empty, say so in writing and recommend heated storage — don't quietly hope.

Safety: ventilate when running the engine, fuel and fogging vapors are flammable, and let manifolds cool before draining.