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What is a Waterbox on a Jet Ski? [Explained]

What is a Waterbox on a Jet Ski? [Explained]

The waterbox on a jet ski is actually a baffling muffler designed to restrict exhaust flow to create back pressure and prevent excessive noise. To this end, the waterbox forces exhaust gases to go through water before they exit the hull. Also, this key feature prevents water from flowing into the cylinders when the jet ski capsizes.

If you want to find out more about the waterbox, you’ve come to the right place. Let’s drill into the details and talk about this lesser-known unit in detail!

What is a Waterbox on a Jet Ski?

Simply put, the waterbox, also known as a water lock box, is basically a muffler on the jet ski that is intended for three main purposes:

  1. Reduces exhaust noise
  2. Prevents water from entering the engine through the exhaust system
  3. Provides adequate back pressure

1. Reducing Exhaust Noise

Due to their high performance, jet skis are extremely noisy, especially supercharged models. Their pump, engine, and exhaust are all very loud, even in stock condition.

To keep the level of noise under the legal limit, manufacturers try to keep the skis’ noise down in every way possible.

This is why every jet ski comes with a waterbox as standard, which acts as a silencer in the exhaust system. As a rule of thumb, the larger the waterbox, the more noise reduction it provides.

Also, the waterbox works as an air pollution control device, as the exaust pollutes the water instead of the air.

2. Keeps Water Away From the Engine

Another important role of the waterbox is to keep water away from the engine.

Since the exhaust system is discharged into the water, there would always be a danger of water entering the cylinders.

This is where the waterbox comes into play, as it blocks water from flowing backward in the exhaust system.

This is especially important when doing tricks like jumping or when the jet ski turns upside down! To keep water away from the engine, if your jet ski capsizes, you have to make sure that you turn it back over in the right direction.

Doing it the wrong may end in a flooded engine as the waterbox can only block the water if the ski is turned the right way.

3. Provides Adequate Backpressure

Last but not least, the waterbox provides the correct back pressure that the engine needs for proper operation. This was very important on vintage 2-stroke jet skis, as their engines required adequate exhaust backpressure to run properly.

Although modern 4-stroke jet ski engines are less sensitive to back pressure, the waterbox is still a key feature on these machines. On 4-stroke skis, the major roles of the waterbox are dampening the noise and keeping the engine from flooding.

How Does a Jet Ski Waterbox Work?

The jet ski waterbox works in a very simple way; it forces the exhaust fumes through water before they exit the exhaust system.

As the name suggests, the waterbox is a metal box filled with water. The exhaust fumes are forced into this water, bubble up, and finally flow out the exit into the water.

In most cases, the waterbox is baffled, and airflow is controlled by the holes in the baffles.

Aftermarket Jet Ski Waterboxes

While factory-installed waterboxes focus on noise dampening and controlling backpressure, their aftermarket counterparts are typically smaller and less restrictive.

Therefore, they have less backpressure, which typically translates to slightly higher performance. As you can imagine, these units are much louder and generate a “dirt-bike like” exhaust sound, especially on 2-stroke skis.

Can You Straight Pipe a Jet Ski?

Straight piping a jet ski means removing its waterbox and replacing it with a “straight pipe.” Although it is possible to straight pipe a jet ski, it’s definitely not recommended as it carries several risks. Without the waterbox, there’s nothing to keep water away from the engine. What’s more, a straight piped jet ski is always extremely loud!

On top of that, the lower backpressure can also result in a lower bottom-end and may cause various malfunctions.

In most areas, modified exhaust systems may be illegal if they are louder than stock. Therefore, to stay safe and legal, it’s not recommended that you remove the waterbox on your jet ski.


The waterbox is the muffler on a jet ski mounted between the engine and the exhaust outlet.

It’s intended to dampen exhaust noises, keep water away from the engine, and produce adequate backpressure.

Removing the waterbox on a jet ski (a.k.a. “straight piping the jet ski”) is not a good idea, as it makes the machine extremely loud and leaves the engine without any protection.