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Fill a bucket with warm water and add in a few squirts of car safe soap. Soak a scratch-free cloth in the soapy water and wipe down the exterior of your jet ski with it. Pay close attention to the bottom of the jet ski, where algae and slime tend to accumulate. Don't use dishwashing liquid or hand soap. Thoroughly rinse off your jet ski with clean water. You can do this with a hose or by filling and dumping buckets of water onto the jet ski. Afterwards, let the jet ski air dry completely. If you want to, you can then use a high-shine protectant car wax to buff and shine your jet ski. Get a bottle of fuel stabilizer and add it to your gas tank as the product label instructs. This will prevent fuel contamination and will also keep residue from building up in the carburetor, fuel injection system, and gas lines. After putting the stabilizer in, top off your gas tank with premium gas. This will prevent condensation from forming on the inside of the gas tank. Spray lubricant on the areas of your jet ski that are often rotating and turning. Some of these areas include the steering nozzle pivot points, the reverse mechanisms, and the brake mechanisms. It's also a good idea to spray the engine as well as the electrical components to rid them of any traces of water.

Summary:
Wash the exterior with car safe soap. Rinse and dry your jet ski. Put stabilizer in your gas tank. Fill up your tank with gas. Lubricate your jet ski's moving parts.