There are two ways to go here. One is a traditional hard case. The best of these turn the housing into an extension of the camera, leaving all the dials and buttons to work just as if they were in your hands. Alas, price can be an issue here.
A case in point is the Aqua Tech DC-5 v2. It’s purpose-built for the Canon 5D, and moreover is purpose-built for surface work or near a surfboard. It’s made of composite material, so is light and very rugged. And you can swap out the lens port to accommodate different lenses. The down side? It's $1,800. Yikes! But if you want to ensure your camera has the best protection and most functionality, this is the real deal.
The alternative is a “soft” case, made from polyurethane. Think of a zipper-type kitchen plastic bag made from more rugged and durable plastic and with a hard lens cover for clear imaging. A good example is the Aquapac SLR Camera Case, model AQP-48 ($140). It’s designed for cameras such as the Canon D5. And it’s a good surfing case—it's waterproof to 15 feet and it floats if dropped. Access to your camera's controls isn’t as good as on a hard case, but it's not bad. It’s also a good case to use when around the beach as it keeps out sand and other nasty stuff.