I bought the SD300 camera and underwater case for a once-in-a-lifetime trip to French Polynesia. The camera is AWESOME. I have spectacular underwater pictures from two weeks of snorkeling off of Bora Bora and Moorea. I would clean the case each night as the very fine sand would get in the grooves and stick to everything. This meant I had to grease the gasket after cleaning the sand off of it. Two weeks of carefully greasing the gasket caused it to stretch so the case no longer closes. The case wasn't useable for the last few days of our trip. I've been searching off and on for replacement gaskets to no avail. Today I sent Canon an e.mail. I'm surprised these aren't easy to find, and since it wasn't hard to stretch it, I'm surprised they didn't provide a spare gasket. Raise the price 20 cents and give us a few spares!
I used Moisture Muncher capsules inside the case to prevent fogging. I assume they worked, I had NO problems with moisture. The little capsules fit in the case no problem and turn purple if full of moisture (also helpful to indicate your case is leaking!) Mine never leaked, not even a trace. I would get 3-4 days of use out of a capsule before it looked purple to me.
You can NOT see the viewscreen at all in daylight while snorkeling. I just pointed and shot and hoped it worked. You can't hear the camera's fake 'click', either, so a lot of shots I THOUGHT I made didn't actually take. A lot of shots I thought were perfect were off center. None were really out of focus, though. It was best in perfectly clear, calm shallow water where the subject is close to you. It was more difficult trying to zoom in to something 20' away as you can't see it on the viewscreen and any movement (bobbing up and down, left and right) makes the far-away object move out of your shot just as you push the button. These probably are not specific to Canon or the case, though.
Great camera. Great case. I had fun using it. The pictures were stunning. Need spare gaskets!