Figuring out how to winterize a jetski is probably the most important thing you'll do for your personal watercraft all year, especially if you live somewhere where the temperature drops below freezing. I know it's tempting to just park it in the garage, throw a tarp over it, and forget about it until May, but that's a great way to end up with a cracked engine block or a fuel system full of gunk. Honestly, spending an afternoon doing this now will save you a massive headache and thousands of dollars in repairs when the weather warms back up.
The whole process isn't actually that complicated once you break it down into a few manageable chunks. It's mostly about keeping moisture out, protecting the internal metal bits from rust, and making sure the fuel doesn't turn into varnish over the next five months.
Start with a deep clean
Before you even touch the engine, you need to give the exterior some love. If you've been riding in salt water, this is non-negotiable, but even fresh water riders should pay attention here. Algae, scum, and those tiny little organisms in the lake can really bake onto the hull over the winter. If you leave that grime on there for months, it becomes incredibly hard to get off later, and it can eventually mess with your gel coat.
Give the whole thing a thorough wash with some soapy water. Use a soft sponge so you don't scratch anything. While you're at it, check the bottom of the hull for any deep scratches or dings that might need a repair kit. Once it's clean and dry, I usually recommend a good coat of wax. It might seem like overkill for a boat that's about to sit under a cover, but it adds a layer of protection against moisture and dust. Plus, it'll look great when you uncover it next season.
Dealing with the fuel system
The gas sitting in your tank is one of your biggest enemies during the off-season. Most gas these days has ethanol in it, which is notorious for attracting moisture. If that water-heavy fuel sits in your lines and injectors all winter, it can cause corrosion or just gum everything up.
The first step in how to winterize a jetski fuel system is adding a high-quality fuel stabilizer. Follow the directions on the bottle—usually, it's about an ounce for every few gallons. After you pour it in, head to the dock or use your hose attachment to run the engine for about 5 to 10 minutes. You want that stabilized fuel to work its way through the entire system, from the tank all the way to the engine.
Some people argue about whether you should store it with a full tank or an empty one. Generally, a full tank is better because it leaves less room for condensation to form on the inside walls of the tank. Just make sure that the fuel that's in there is treated.
Flushing and draining the water
This is the part that keeps most owners up at night. If there is water trapped inside your engine's cooling jackets and that water freezes, it expands. Since metal doesn't like to stretch, the engine block will literally crack. That's a "game over" scenario for your jetski.
If you have a modern four-stroke, you'll usually hook up a garden hose to the flush port. Start the engine first, then turn on the water (never turn the water on before the engine is running, or you might flood the cylinders). Let it run for a few minutes to clear out any salt or sand.
After you shut the water off (and then the engine), you want to get as much of that residual water out as possible. Some people use an air compressor to gently blow out the lines. Another popular trick is to use a bit of RV antifreeze. It's non-toxic and ensures that any tiny pockets of water left behind won't freeze and cause damage. Just be sure to use the pink stuff (RV/Marine) and not the green automotive kind, which is toxic to the environment.
Fogging the engine
This sounds a bit technical, but it's basically just spraying a specialized oil into the engine to coat the moving parts. When an engine sits for months, the oil drains down to the bottom, leaving the cylinder walls and valves exposed to the air. Humidity in the air causes surface rust, which can lead to a loss of compression later on.
To do this, you'll usually need to remove the air brand/filter and spray "fogging oil" directly into the intake while the engine is running (briefly!) until it starts to stall or produce a lot of white smoke.
If you're not comfortable doing it while it's running, or if your specific model makes that difficult, you can pull the spark plugs. Spray a healthy dose of fogging oil into each spark plug hole, then tap the start button for a split second just to move the pistons up and down. This spreads the oil around the cylinder walls. While the plugs are out, take a look at them. If they look fouled or black, now's a great time to buy a new set so you're ready for spring.
Don't forget the battery
Batteries and cold weather are a terrible mix. If you leave your battery in the jetski and the temperature drops, it will slowly lose its charge. A dead battery can actually freeze and crack, which is a mess you don't want to deal with.
The best move is to take the battery out entirely. Store it in a cool, dry place—like a basement or a heated garage—on a piece of wood or a shelf (avoiding direct contact with concrete floors is an old-school tip that still holds some weight).
Invest in a "battery tender" or a smart trickle charger. These devices monitor the battery's voltage and only charge it when it needs it. If you just use a regular old charger, you might overcook the battery and ruin it. A tender will keep it healthy and ready to go the second you drop the ski back in the water.
Protecting the hull and interior
Once the mechanical stuff is done, you've got to think about the physical storage. If you're storing it outside, a high-quality, breathable cover is a must. Don't just use a cheap blue tarp from the hardware store; those can trap moisture underneath and lead to mold and mildew on your seats.
Inside the footwells and the engine compartment, I like to use a bit of WD-40 or a specialized silicone spray. Lightly coat the metal components, hose clamps, and any exposed wires. This acts as a moisture barrier.
Also, consider the "critter factor." Mice and rats love to climb into engine bays to build nests during the winter. They'll chew through wires and seat foam faster than you can imagine. Some people swear by putting dryer sheets or mothballs inside the storage compartments to keep them away. Just remember to take them out before you ride next year!
Proper positioning
Lastly, when you park the trailer, try to keep the nose of the jetski slightly elevated. This allows any condensation or leftover water to drain out of the back through the drain plugs. Make sure you actually take the drain plugs out. I've seen people leave them in, only for the hull to fill up with rainwater or melted snow that leaked through a hole in the cover.
If you can, try to keep the seat propped open just an inch or two. This allows for airflow and prevents that funky "old basement" smell from developing inside the engine compartment.
Learning how to winterize a jetski might feel like a chore the first time you do it, but after a couple of years, it'll just be part of your routine. It's all about protecting your investment. These machines aren't cheap, and taking these steps ensures that when that first warm Saturday in May rolls around, you'll be out on the water instead of waiting three weeks for a spot at the repair shop. Take your time, follow the steps, and you'll be golden.