we have had discussions on this. usually it is a safe estimate, which if you go beyond maybe a few metres it should be fine. but, you may have problems with buttons jamming due to over pressure. i had a canon A40 which housing rated was to 100ft i.e. 30m or so, i had it down to 40m and it came back alive. BUT, i didnt take photos on that depth.
if you want your insurance to work for you if anything happens, it would be more advisable to keep to manufacturer's estimate.