Winterizing your boat is one of the most critical tasks you’ll perform as a boat owner. Done right, it protects your investment and ensures you’re ready to hit the water when spring arrives. Done wrong? You could be facing thousands of dollars in repairs—or worse, a boat that won’t run at all.

After 41 years of marine assistance across Canada, the C-Tow team has seen it all. We’ve responded to countless spring emergencies that could have been prevented with proper winterization. So before you button up your boat for the season, let’s talk about the most common—and costly—winterization mistakes boaters make.

Don’t Skip the Engine Flush

One of the biggest mistakes boaters make is failing to properly flush their engine with fresh water before storage. If you’ve been boating in saltwater or even freshwater with sediment, contaminants can corrode your engine over winter.

The mistake: Running your engine dry without flushing, or doing a quick 30-second rinse and calling it good.

Why it matters: Salt crystals, algae, and mineral deposits will sit in your cooling system all winter, causing corrosion and blockages that can lead to overheating come spring.

Do this instead: Run fresh water through your engine’s cooling system for at least 5-10 minutes. Use muffs or a flushing port, and make sure the water flows freely from the exhaust. Then add antifreeze designed for marine engines according to your manufacturer’s specifications.

Don’t Forget to Stabilize Your Fuel

Gasoline breaks down over time, forming varnish and gum that can clog fuel lines, carburetors, and injectors. Yet many boaters simply top off the tank and walk away.

The mistake: Storing your boat with untreated fuel, or adding stabilizer but not running the engine long enough to circulate it through the system.

Why it matters: Degraded fuel is one of the top reasons boats won’t start in spring. You’ll be looking at costly fuel system cleaning or replacement.

Do this instead: Fill your tank to about 95% capacity (leave room for expansion), add a quality marine fuel stabilizer, then run the engine for 10-15 minutes to ensure the treated fuel reaches every part of the fuel system. This applies to both inboard and outboard engines.

Don’t Leave Water in the System

Water expands when it freezes—and that expansion can crack engine blocks, split hoses, and destroy pumps. This is the most expensive winterization mistake you can make.

The mistake: Assuming you’ve drained all the water when you haven’t checked every system, or thinking “it doesn’t get that cold here” and skipping this step entirely.

Why it matters: Even in milder Canadian climates, overnight freezes can cause catastrophic damage. A cracked engine block can cost $5,000-$15,000 to replace.

Do this instead: Drain and antifreeze every water system on your boat—engine cooling system, raw water intake, livewell, washdown pump, head, galley, and any other plumbing. Don’t forget the hot water heater and air conditioning system if you have them. Use non-toxic marine antifreeze (the pink stuff, not automotive antifreeze) and pump it through until you see it coming out of every faucet and drain.

Don’t Neglect Your Batteries

Batteries left connected or uncharged over winter will sulfate, lose capacity, and potentially freeze if they discharge too much.

The mistake: Leaving batteries connected to the boat, storing them on concrete floors, or putting them away without a full charge.

Why it matters: A dead battery in spring is frustrating. A ruined battery that needs replacement is expensive and avoidable.

Do this instead: Fully charge your batteries before storage. Disconnect them and remove them from the boat if possible. Store them in a cool, dry place (concrete is actually fine—that’s an old myth). Better yet, connect them to a quality marine battery maintainer that will keep them at optimal charge all winter without overcharging.

Don’t Use Automotive Antifreeze

This one seems obvious, but it happens more often than you’d think—especially with DIY winterizers.

The mistake: Using green automotive antifreeze instead of pink non-toxic marine antifreeze in your plumbing systems.

Why it matters: Automotive antifreeze is toxic and can contaminate your freshwater system, making it unsafe for drinking, cooking, or washing. It’s also not formulated for the same applications.

Do this instead: Always use non-toxic, propylene glycol-based marine antifreeze (usually pink) for your plumbing, head, and galley systems. Use ethylene glycol-based antifreeze only in closed cooling systems where it’s specifically called for, and never where it could enter your freshwater supply.

Don’t Leave Your Boat Uncovered or Improperly Covered

Your boat needs protection from the elements, but the wrong cover can actually cause more harm than good.

The mistake: Using a tarp that doesn’t breathe, leaving gaps where snow and rain can enter, or creating a cover that pools water instead of shedding it.

Why it matters: Trapped moisture leads to mold and mildew. Pooled water adds weight that can damage your boat. Snow accumulation can collapse covers or even damage the hull.

Do this instead: Invest in a quality, breathable boat cover or shrink wrap that’s properly installed with adequate support structure. The cover should shed water and snow, not collect it. Make sure there’s ventilation to prevent moisture buildup, but no gaps large enough for pests to enter.

Don’t Forget About Rodents and Pests

Mice and other critters love boats as winter homes—and they can cause shocking amounts of damage in just a few months.

The mistake: Assuming your boat is sealed up tight, or thinking “it’s never happened before so it won’t happen now.”

Why it matters: Rodents chew through wiring, upholstery, hoses, and insulation. They nest in engines and leave droppings everywhere. The repair bills can easily reach thousands of dollars.

Do this instead: Remove all food, paper products, and fabric items that might attract pests. Place rodent deterrents throughout the boat—dryer sheets, peppermint oil, ultrasonic devices, or traditional traps. Block any openings where pests might enter, but maintain ventilation. Check your boat periodically through winter if possible.

Don’t Skip the Oil Change

Some boaters figure they’ll change the oil in spring before launching. That’s actually backwards.

The mistake: Storing your boat with old, contaminated oil in the engine.

Why it matters: Used oil contains acids, moisture, and combustion byproducts that can corrode engine components over the long winter months.

Do this instead: Change your engine oil and filter before winter storage, not after. Fresh oil provides better protection during storage. Run the engine briefly after the oil change to circulate the new oil through the system.

Don’t Ignore Your Outboard’s Lower Unit

The lower unit is easy to forget, but it needs attention too.

The mistake: Not checking or changing the lower unit gear oil before winter.

Why it matters: If water has contaminated your gear oil (which you’ll see as a milky appearance), it can freeze and expand, cracking the housing. Even without water intrusion, old gear oil doesn’t protect as well during storage.

Do this instead: Drain and inspect your lower unit gear oil. If it looks milky or has metal particles, you may have a seal problem that needs addressing before storage. Refill with fresh gear oil to manufacturer specifications.

Don’t Leave Valuables or Electronics Onboard

This isn’t mechanical, but it’s still important.

The mistake: Leaving expensive electronics, fishing gear, safety equipment, or personal items on the boat all winter.

Why it matters: Theft happens, even in secure storage facilities. Temperature fluctuations can also damage sensitive electronics. And if you need that safety equipment for another vessel or emergency, you won’t have access to it.

Do this instead: Remove all valuable electronics, fishing equipment, life jackets, flares, fire extinguishers, and personal items. Store them in a climate-controlled environment. This also gives you a chance to inspect safety equipment and replace anything that’s expired.

Don’t Forget to Document Everything

One final mistake: not keeping records of what you did and when.

The mistake: Relying on memory about which winterization tasks you completed.

Why it matters: Come spring, you won’t remember if you added fuel stabilizer, whether you pumped antifreeze through the head, or when you last changed the lower unit oil. This can lead to missed steps or duplicate work.

Do this instead: Create a winterization checklist and document each task as you complete it. Note the date, products used, and any issues you discovered. Take photos if helpful. Keep this with your boat documents so you have a reference in spring and a maintenance history for future years.

The Bottom Line

Proper winterization isn’t just about protecting your boat—it’s about protecting your investment, your safety, and your peace of mind. The few hours you spend doing it right will save you countless headaches and potentially thousands of dollars in repairs.

At C-Tow Marine Assistance, we’ve been helping Canadian boaters for over four decades. We’ve seen what happens when winterization goes wrong, and we’ve helped countless members get back on the water after winter damage. Our advice? Don’t cut corners, don’t rush, and when in doubt, consult your owner’s manual or a qualified marine technician.

Your boat has given you a season of memories on the water. Give it the care it deserves so it’s ready to do it all again next year.