If you seal it completely it will be very positive. Personally I really dislike anything positively buoyant, esp. a camera. You can never set it down or turn it loose if you need both hands. You should have it tethered to you so you can go hands free when you need to and when you do release it, it floats up and tangles up with your hoses. If it were mine, I would make the PVC into open compartments and then seal one or more to make it just barely negative in salt water. Odds are it will be about right if you leave the entire thing open. Foam does not work well due to it compressing as depth increases. If you need to add back a little positive buoyancy, you can add some non compressible material into the PVC pipe or install a section of foam then seal the ends with glue, epoxy or a similar material. If you add back floatation, be sure to add it back in an even manner, otherwise one side of the camera will be more positive and want to twist the camera as you hold it making it very uncomfortable to control over a long dive.