Panasonic sells a housing for the TS3 to be able to go deeper - but trust me - you won't even like the results at 15m !!!
Using the flash will simply light up all the particles and bubbles, making your subject the background. Messy.
To be able to do nice pics further down, you need a strobe or a very strong LED flashlight to light up the subject. Then a housing for whichever camera you want to use.
Higher end cameras with interchangeable lenses let in more light than the tiny camera, thus a better picture.
Within your budget - I just bought from SpyderTC a nice starter kit:
http://www.scubaboard.com/forums/cl...397435-olympus-sp-350-camera-accessories.html
For anything below 10m you'll want to use an UW flashlight to light the subject so your camera can focus on it.
Another for sale, more expensive though :
http://www.scubaboard.com/forums/cl...ea-sea-housing-lens-port-extras-included.html
Even though the Canon 300d has a "lower" megapixel CCD than the Olympus I just bought, the Canon has better optics. Thus bigger, bigger and more expensive housing.
I didn't buy the Canon because that kit still needs a strobe for deep dives, extra costs, and was over my budget of this year.
Buying used is a great investment, as you can resell after 2-3 years for at least half of what you bought it for.