Canon Powershot S1 IS with WP-DC20

Hello, I'm new both to digital and underwater photography and I would like to buy Canon S1 IS with WP-DC20 housing.

Any of you has used this equipment?

Does the housing allow the use of all the functions?

Does the 10X zoom work well underwater?

Is it possible to add a strobe and/or additional lenses (Wide angle/macro)?

I would like to buy a rather simple and inexpensive system since I'm only an amateur and I can dive only one or two weeks a year, and most of the time I will use my camera on land.

Thanks for your help!!