The problem with Canon P&S's, is their slow AF shutter lag. They won't make them compete with their DSLRs. This camera has a full AF shutter lag of 0.6 to .65 seconds, plus it gets longer when using a flash.
While this is much faster than oly or nikon P&S cams, it is painfully slow underwater, when the perfect composition is gone before it will trigger.
The class leaders in this area are Panasonic and Sony. They have lots of P&S cams with full AF shutter lags in the .3 - .35 second range.
If it were me, I'd go with a sony that has an ike housing and manual exposure, like the hx1 or hx5.
Or I'd go with a pany, and the LX5 would be truly a great P&S camera! Good IQ, and very good AF performance and total lag time.
One of the cheaper panys like the zs5 of zs7 would be very decent alternatives - they offer A, S, and M exposure modes, and ikelite housings.