There are several underwater housing for Canon SX20 Camera. Most of them are soft waterproof cases and only one hardshelled case which is Ikelite Underwater Camera Housing more expensive than camera it self !
If you are going to buy one of those soft Canon SX20 underwater housing then you have to be very careful when you use it. small damage on underwater housing can ruin your Canon SX20 camera. Canon do manufacture underwater housing for point and shoot cameras but they don't have one for Canon SX20 or Canon SX10 cameras. All these Canon SX20 underwater housings work for Canon SX10 and Canon SX1 IS cameras too.
SX20 Ikelite Underwater Camera Housing is perfect in everyway except when it comes to price. molded of crystal clear polycarbonate for corrosion free operation up to a depth of 200ft (60m) and Every camera function is accessible. The housing comes standard with an optical grade glass flat port which allows full use of the camera's zoom range without vignetting. Only problem is this Ikelite Underwater Housing is way too expensive ! Amazon has the lowest price you can find but still even that's too much !
This SX20 waterproof Housing is only $30. For universal use with a wide range of cameras; Protect expensive cameras, flashes and lenses from bad weather and harsh conditions, such as rain, snow, dust, grit and sand. But I'm not sure How waterproof is this casing. This is not a underwater housing.
This waterproof case is only $70. Waterproof 20 meters; dedicated soft German TPU, tensile strength, can be a buffer, drop resistance protection. No need to worry about falling into the water, it can float. Reliable double seal closing system with waterproof zipper and Roll & Velcro opening.
This Canon Waterproof Case WP-DC20 is made for Canon S1 camera. But I saw someone say this waterproof camera case fits fine for Canon SX20 and Canon SX10 IS cameras. If you can find this in your local camera store take your Canon SX20 there and see if Canon Waterproof Case WP-DC20 fits for your camera. Then you can buy it there or from Amazon.com depending on where its cheaper. Canon Waterproof Case WP-DC20 is $210 at Amazon.com.
or cheaper one like 