Stay away from those zip-lock looking bags.....
As mentioned earlier, both Canon and Sony make plastic housings for their cameras. They range in price from $150-$400 and generally work fairly well for quick pictures. But don't expect to get much closer than 18" to the subject and that little flash in your camera can't put out enough light to light up that entire shark. They can be fun though, if you understand their limitations.
You can bump their performance up a notch by getting an underwater strobe (a water proof external flash) and attaching it to your camera. Sea & Sea and Ikelite make small inexpensive ($200+) strobes that can mount to your point and shoot camera housing. If you are comfortable with setting exposure levels and f-stops, then you can create some really nice photos using a small digital camera and a small strobe.
If you want to get serious about doing underwater pictures, plan on spending $3000-$5000 to get started using some of the new 8-10Mpixel DSLRs that are on the market. It's like fish tanks... do you want to keep a Niger Trigger in a 29 or do you want a reef in a 300?
If your goal is to have fun, get a housing and play. The nice thing about digital is that you get to see your mistakes immediately!