The only PowerShot models intended for underwater use are the D series (e.g. the D20 or D10) and those are waterproof for shallow depths such as swimming pools or snorkeling -- but not deeper depths such as scuba diving.
Canon does make underwater enclosures for MANY PowerShot models. These enclosures are generally safe for any recreational scuba diving depth (e.g. 30-40 meters or 100-140 feet) provided you follow the appropriate care when using the enclosure (this is one product where you definitely want to know and follow the instructions... it's very important that you don't allow anything to compromise the seal.)
You can find a list here: http://www.usa.canon.com/cusa/consumer/standard_display/underwater_photo