Taking your Pentax K3 into the ocean is not the usual photographic recommended exercise, but this type of ocean/water photography is what I love and the tough Pentax K3 is ideal.
All my shots are taken with the Pentax K3 in a Dave Kelly custom made water housing with custom knobs and buttons to change the speed, F stops and ISO in the surf if needed. Focus is manually set with the camera on manual with two housing ports available: one Dome Port for the Pentax 10-17mm fish eye and one Flat Port for the Pentax 70mm or 18-55mm Pentax Lens. A typical manual setting for surf photography will be 1000/F8 ISO 200 which will capture a fast moving surfer whilst still give a good depth of field on the fish eye, set at around 10-12mm. The manual focus is set for around a dangerous three feet as you have to get in close and personal with the fish eye. (I sticky tape the lens focus). With the longer lenses, you keep them as dry as possible and I often take shots off a surfboard or paddleboard from a distance.
A hassle in the ocean is water drops on the ports, so I lick the port with saliva (yuck) to get it free of drops and duck into the water prior to every shot.
Another type of ocean photography is the specialised underwater wave photos and the split water above and below water shots, all these have there own best settings with lots of experimentation to get keeper shots. Here are a few different examples of my water shots as well as a photo of the K3 in water housing with a link to my web page for further photos and shots of my bloody expensive obsession:
Hewysurf | Surf Photographer