I have a Pentax, my wife has an Olympus.
We are both happy with our cameras.
Pentax has more zoom (5x), a very good menu system and uses SD cards.
Olympus is specially built to be shock resistant and has a metalcover that automatically covers the lens when off. Zoom is the standard 3x, but cards are the XD type which are less available.
The pentax LIVES in my cargo pocket when I am not at work and takes knocks regularly and is still working fine after 2+yr though focus at infinity is a little spotty.
I use an 'Always On' camera cover and have a neck lanyard rather than a wrist strap for it with a cinch so I can go snorkling without constantly having it in hand.
Both Pentax and Olympus have been producing waterproof digitals for more than 5yr, several years longer than the other major brands. For this reason and general happiness with the cameras we have I would reccommend either brand over all but the specialty makers like Sealife that build for diving
as opposed to snorkeling.
For the better zoom and the more common memory card I would reccommend Pentax slightly higher than Olympus. As far a shock resistance, electronics
are almost disposable items these days so I personally don't count on more than 3-4yr from a camera.