You did not say why you want a waterproof housing. If it's for shallow swimming, surfing or kayaking, you might be OK with one of the $30-40 vinyl bags. These keep the sand and water out if you are not too rough on them. When you go deeper, they don't protect the camera and the screen will crack. You would need a hard case rated for depth.
I looked at your Seashell, interesting. I wonder if you would have access to the back buttons on the S90, you would need that for full control. But the price of $120 is interesting... But the Canon case is just a bit more expensive ($170) and gives access to the rear buttons. To me it's a better match. I'd consider the Seashell only for a camera I could not get a dedicated housing for.
For serious work, the Canon and Ikelite are good. There are two other companies who make metal housings too, very nice (Fisheye FIX and 10Bar). If you want pro grade, consider the metal housings. They give you access to the rear wheel, which makes for faster control changes. But for most of us the Canon or Ikelite are just fine.
The Canon DC35 case works well, but does not access the rear wheel. That means more steps to change settings, that's all. Ikelite makes a good case too, for a bit more. It also does not access the rear wheel.
If you want to add a strobe, the Ike may be a better start, since it comes with a tray and arm. Their AF35 works well. There is a very cheap strobe, the Intova ISS2000, that also works well. There are so many options with strobes I can't really recommend one, it so depends on your needs and your wallet.
Again, the choices are wide, depending on your use. There is a very active discussion of case options on another board, Scubaboard.com. Check out the Canon Forum.