Google or other search engine will bring you tons of hits. You can find information about your target camera or where to buy it, but you may need to better focus your search to find these sample pictures.
There are several websites, e.g., Steve's Digicams, Digital Photography Review,
or Trusted Reviews, which provide comprehensive reviews of many camera models including test pictures. These pictures demonstrate the camera features and focus on technical issues like resolution or ISO sensitivity. So, you may also like to search for pictures shot by more casual camera users.
Paddlers are posting links to their pictures at popular forum like paddling.net. There are also many posts about about "water" or "paddling" cameras. So, participate in the discussion or search archives.
It is worth to visit forums at Digital Photography Review. They have discussions on every camera brand. Here is Pentax Talk.
Paddling blogs include usually a lot of pictures. You can see your camera in action and different approaches to paddling photography there. It may require some digging or asking questions since bloggers not always provide technical details about their photo gear. Remember: bloggers love questions and comments!
Most pictures displayed in my two paddling blogs, Paddling with a Camera and Fitness Paddling, were shot with Pentax Optio W10 during last two years. Earlier, I used the original Optio WP. Occasionally, you can see pictures shot with Canon EOS 10D.
Amazon displays customer images for cameras and other products. Here is e.g., Pentax Optio W30 gallery. You can add your pictures there.
Flickr.com provides many interesting photo resources and tools including
camera finder. Waterproof cameras are not really popular, so you need to do some digging there. Here are some examples what you can find: Pentax Optio W10
Olympus Stylus 720 SW,
770 SW, and
Only "interesting" pictures are displayed there. They may be not available for new or less popular camera models.
You can also find many Flickr groups devoted to specific camera models with pictures and often discussion forums. Here are groups for some waterproof cameras:
Pentax Optio W30, W20W10,
Olympus Stylus 720 SW, 770 SW,
and GoPro Hero.
Do you more suggestions where to find sample photos for a specific camera model?