Using 'all weather' housing for Canon IXUS i (SD10) underwater

I'm thinking of getting a Canon IXUS i (SD10 in some countries) with
its 'all weather' waterproof housing (AW-DC10). The housing is
advertised as being rated to 3m. I want to use it snorkelling, where I
might get down to 5m (or maybe a little more). Anyone know what
happens to these devices when the rating is exceeded? Do the buttons
simply not work, or is it likely to actually leak? Would I better off
getting one of the soft case housings (which are about the same price,
but not as neat).