Both are interesting cameras. The Pentax Optio W20 (about US$299; www.pentax.com) and Olympus Stylus 720SW (about US$399; www.olympus-global.com) are seven-megapixel cameras that are extremely compact and have large two-and-a-half-inch LCD screens for viewing (alas, no viewfinder). Both have three-power zoom lenses (meaning the ratio between the wide" and tele" settings is 3:1, which is about average). Both are also designed to be rugged and waterproof, not simply water-resistant.
That said, the Olympus comes up a little better in the wet department. Its waterproof down to ten feet, i.e., you could actually go down that deep and shoot pictures with it. Thats pretty amazing, when you think about it. The Pentax, on the other hand, is good down to five feet, which is still pretty darned good. Any camera that goes deeper than five feet on a whitewater trip probably has bigger problems than whether a little water might leak in.
So it might come down to what feels better to you. Some people have complained that the Olympus can be a little difficult to manage if you have thick fingers. So get both cameras side by side in a camera store and try taking a few pictures with each, run the zoom controls, figure out how easy/hard it is to delete or edit pictures, things like that. Ive found the software and photo-management features of a digital camera count for more than some traditional camera features, such as zoom length.
Also, Im not wild about cameras that lack a traditional viewfinder. Running the LCD screen is a big drain on batteries. Plus, I simply dont think you see" the picture as well as you do when peering through a viewfinder, which tends to concentrate the mind a little more. Yet one more reason why people are taking more pictures than ever, yet theyre probably lousier than ever. But I digress
Check out this years more than 400 must-have gear items, including a comprehensive camera section, in the 2006 Buyers Guide.