Capture scenes like this using an underwater housing for your Canon.
Underwater housings let snorkelers and divers snap away using a standard digital camera. Canon doesn't make an underwater case for its PowerShot SX100 IS superzoom model, but customized housings are available from third-party accessory manufacturers.
The Market
Your choice of underwater housing units designed to work with the PowerShot SX100 IS comes down to two products: the cheap-and-cheerful DiCAPac WP-610, and the much more expensive and sophisticated Ikelite 6148.10. The former is basically a flexible vinyl bag that is waterproof to 16 feet, while the Ikelite's tough polycarbonate body is good for 200 feet.
Your Diving Habits
Do you dive often enough and deep enough to justify the much greater cost of the Ikelite? Or are you a fair-weather type who takes the plunge only once or twice a year while on vacation? If you're the latter, then you'd be better off with the DiCAPac.
Other Things You'll Need
You may require filters to deal with certain visibility conditions. To get the best out of the Ikelite, you'll need an optional external strobe light that reveals the brilliant colors of marine life in the murky waters below 30 feet. Otherwise your fish photos will be more "oh no!" than Nemo.
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