How do I prevent condensation in my waterproof GoPro casing?

If there is room inside the case a good option would be to place a silica gel pack inside. The silica gel will lower the moisture level inside the case, which will help to prevent condensation.

You can also get reusable packs that can be baked to renew. Look for "indicating silica gel" (Example).

This is pretty common for the GoPro underwater housing. One of the larger GoPro dealers actually has custom made inserts that help with the issue. They call them GoPro Anti-Fog Inserts. This is the description:

Prevent your camera's housing from fogging, even in cold temperatures/humid environments. Great value and performance: includes 3 sets of anti-fog inserts. Each set lasts 4-5 uses and can be dried in an oven (3 minutes) and used again.

In my personal experience, this is just something you learn to live with for the GoPro. It isn't perfect, but I just try to acclimate the unit to the water for a time before my adventure. I've done snorkeling, scuba, swimming, and rafting this way. If you are having trouble with the outside lens fogging up, wipe some Vaseline to prevent that issue.

The official anti-fog inserts work well, but are easy to lose during a quick change. And never at an easy time to replace them. At a fix, I've overcome fogging on a number of adventures in South East Asia by simply tearing off strips of tissue paper. For a recent dirtbike trip up into the mountains of Cambodia, the fogging got particularly bad, and we were out of inserts. We ended up simply tearing up some of the local currency and packing the front, sides and back with it.

Not an ideal fix, but it worked, presumably with the conditions being such that the moisture vapour was more readily absorbed into the porous material then they were to condense on the lens dome of the case.

A more affordable alternative is to use the third-party anti fog inserts. I've bought a pack of FogFree for my old HERO2 and still use the same inserts almost 2 years later. They work the same as the GoPro branded ones but are much cheaper. Just "bake" them to dry and you can keep reusing them.

The fridge/air-con trick never worked for me.

Yes, you can buy the inserts. They aren't great. Your best bet is to cut a paper towel down to 3/4" strips by 2-3 inches long. LIGHTLY dust it with cornstarch and shake off. Fold it tightly like a rectangle. Insert appropriately. I also use an anti-fog spray or toothpaste on the inside of the case. Don't be a nimrod. Buff it out until it's clear. You won't have any issues. I also have been known to soak my camera (in the waterproof case) in whatever water I am getting in. It tempers the case and camera, therefore minimizing condensation.

I found that using a dry(as opposed to wet) face tissue paper inserted in the gaps between the waterproof housing, and the actual camera works wonders and doesn't cost as much as the other action camera focused desiccants.

What can I do to prevent fogging within the GoPro case when I take it out on the water?

The GoPro line of cameras has evolved quite a bit since this question was asked six years ago. The current crop of GoPros are all waterproof, so there's no need for a waterproof case unless you plan to take your camera diving. Without a case, condensation is only a problem if the camera is cold and in moist air, and in that case the easiest thing to do is to just wipe the lens or dip it in the water.

have a look at my article you may find it helpful, it covers how and why condensations occurs, what you can do to reduce or eliminate it.

http://underwaterwillett.com/housing%20condensation.html