How to Winterize a Pontoon Boat (Engine, Plumbing, and Tubes)

What's the checklist to winterize my pontoon's outboard, water system, and pontoons?

Winterizing a pontoon is really about one thing: getting every drop of water out of places where it can freeze, expand, and crack metal or plastic. A pontoon has three separate freeze targets — the outboard's raw-water cooling passages, the onboard plumbing (water tank, pump, livewell, washdown), and the aluminum tubes themselves, which can trap condensation or leaked-in water. Outboards are raw-water cooled (lake water runs straight through and must be fully drained by storing the engine vertical — not filled with antifreeze), while fresh-water plumbing lines do get pink -50F non-toxic antifreeze. Add stabilized fuel, fogging oil, fresh gearcase lube, and a vented cover, and you've covered 95% of what kills boats over winter.

ℹ️ Reference only: For general reference only. This guide does not guarantee any result — every home is different. Verify against your local building codes and a licensed professional before acting, especially for electrical, gas, plumbing, structural, or roof work.

💵 $60-$150 DIY (stabilizer, fogging oil, gear lube + gaskets, a few gallons of pink antifreeze, battery maintainer); $300-$600 at a marine shop for full engine + plumbing winterization, more with shrink-wrap ($15-$25/ft) or generator service. ⏱ 2-4 hours DIY for a single outboard pontoon with basic plumbing, plus cover/shrink-wrap time. ● Use caution

Safety: Fuel and engine work involves explosive fuel vapor — work outdoors or in a well-ventilated space, no open flame or sparks, and use only ignition-protected marine electrical components in fuel/engine areas. Always connect water (muffs or in the water) and turn it ON before starting the outboard; running it dry destroys the water-pump impeller in seconds. Running the engine risks carbon-monoxide buildup; never run it in a closed garage. If you store the boat in the water with shore power, electric shock drowning (ESD) is a real hazard — never let people swim near a powered dock, and have AC/galvanic isolation checked by an ABYC tech. Pink plumbing antifreeze (propylene glycol) is low-tox but still keep it off skin and away from pets; never use toxic automotive ethylene-glycol coolant in potable lines. Lift and block the boat safely if working under it.

Common causes

Trapped water freezing in the outboard's raw-water cooling passages and powerhead, cracking the block or exhaust housing — the single most expensive winter failure. (most common)
Quick check:
Water left in the freshwater plumbing (tank, pump, lines, livewell, washdown) splitting pump housings and fittings. (common)
Quick check:
Untreated ethanol fuel phase-separating and gumming carbs/injectors over a long sit. (common)
Quick check:
Condensation or rainwater trapped inside the aluminum tubes (failed bilge/transom drain plugs, leaky weld seam) freezing and stressing seams. (less common)
Quick check:
Battery left connected and discharged, freezing and cracking the case. (less common)
Quick check:

How to fix it

Stabilize the fuel first. Top off the tank to ~95% (reduces condensation/airspace) and add marine ethanol-rated fuel stabilizer at the label dose. Then connect flushing muffs (or a flush adapter) and TURN THE WATER ON BEFORE STARTING — never run an outboard dry even for a few seconds; the rubber water-pump impeller burns up almost immediately and the powerhead overheats. Run 10-15 minutes so treated fuel reaches the carbs/injectors. Use a marine-rated stabilizer, not just any automotive product.
Fog the engine. With the engine warm and running (water still flowing), spray fogging oil into the intakes (or via the dedicated fogging port on EFI/DI Mercury/Yamaha engines) per your engine manual; carbureted engines typically fog until they stall, EFI/DI engines may not stall, so follow the manual. This coats cylinders and bearings against corrosion. Spark-plug-hole fogging is the alternative on engines without a port.
Drain the raw-water cooling system completely. Shut off and disconnect water, then store the outboard in the full vertical (down) position so all lake water drains out of the gearcase and powerhead by gravity — this is the critical anti-freeze step for a raw-water-cooled outboard. Do NOT rely on pink antifreeze here; the goal is a fully empty, self-draining lower unit. Storing the engine tilted UP can trap water in the gearcase and crack it, so store it DOWN. Note: outboards are raw-water cooled — there is no closed/fresh-water cooling loop to fill (that is an inboard/sterndrive feature, not a pontoon outboard).
Change the gearcase (lower unit) oil. Drain via the lower drain screw, inspect the oil: milky/cloudy means water intrusion and a failing seal — address before spring. Refill with the manufacturer-specified marine gear lube and replace both drain and vent gaskets. Doing this in fall prevents trapped water in the gearcase from freezing.
Winterize the freshwater plumbing. Drain the freshwater tank, water heater (if any), livewell, and washdown. Pump non-toxic -50F propylene-glycol (pink) RV/marine antifreeze through the system: pour into the tank or use a pickup hose, run the pump until pink fluid comes out every faucet, the livewell fill, and the washdown outlet. Drain low points and leave the antifreeze in the lines. Pink antifreeze is for potable/plumbing lines only — never put it in the raw-water engine.
Clear and protect the pontoon tubes. Pull every drain plug on the tubes and the underdeck/bilge area and let any trapped water out; store the plugs where you'll remember to reinstall in spring. Confirm tubes are dry and seams aren't leaking. Leave drains open under a cover so condensation/meltwater can escape rather than pool and freeze.
Pull and tend the battery. Disconnect (negative first), remove it, top up electrolyte on flooded types, charge fully, and store on a maintainer in a cool, dry place above freezing. A fully charged battery resists freezing; a dead one cracks. Reconnect/charge periodically over winter.
Lube, fasten, and protect. Grease steering, tilt/trim, and pivot fittings; wipe down and apply corrosion inhibitor to electrical connections. Use only marine, ignition-protected components and ABYC-compliant wiring near fuel/engine areas — do not substitute automotive parts.
Cover with ventilation. Clean and dry the deck, furniture, and carpet (mildew loves trapped moisture). Use a vented, snow-rated cover or shrink-wrap with vents; if storing in water (not recommended in freezing climates), keep a working bilge pump and de-icer/bubbler running. Trailer storage with the bow slightly high so water sheds off is best.

DIY or call a pro?

Strongly DIY-friendly for a competent owner — fuel stabilizing, fogging, draining the raw-water system, plumbing antifreeze, tube drains, and battery care are all straightforward with basic tools. Hand it to a marine shop if the lower-unit oil comes out milky (seal/bearing work), if there's a generator or onboard heater, or if you simply want a documented annual service. Fuel-system and any AC/shore-power work near fuel vapor should go to a pro if you're not confident.

Tools & parts

Marine ethanol-rated fuel stabilizer
Fogging oil (or engine-specific fogging port)
Marine gearcase lube + drain/vent gaskets
Non-toxic -50F propylene-glycol (pink) RV/marine antifreeze, a few gallons
Flushing muffs or flush adapter (with a water source ready before starting)
Marine grease for steering/tilt/trim fittings
Corrosion inhibitor / fogging-safe protectant
Battery maintainer/trickle charger
Basic sockets/screwdrivers, drain-plug wrench, oil pan, funnel
Vented storage cover or shrink-wrap with vents

Keep a record of every fix you make — what broke, what it cost, how you solved it.

Track your home's fixes in Home Story →

Based on: BoatUS / BoatUS Foundation; American Boat & Yacht Council (ABYC); U.S. Coast Guard / USCG Auxiliary; National Marine Manufacturers Association (NMMA); NFPA (National Fire Protection Association); Mercury Marine service guidance; Yamaha Outboards service guidance

General marine-maintenance guidance, not a substitute for a qualified marine technician or surveyor. Boats and conditions vary; for fuel, electrical, fire, or structural issues — or anything safety-critical — consult a professional. Always follow your engine and equipment manuals.