The Olympus TG-6 is the same size as the TG-5 and has very minor improvements which you can read about in the link above. The TG-6 will work in the TG-5 housing. This section applies equally to both the TG-5 and the TG-6. Either of these cameras can also be considered the best waterproof camera, outside of a housing. The TG-5 is just as good as the TG-6, and we have a great deal on it.
Action cameras like GoPros have their advantages too though. They're generally much smaller than waterproof compacts, making them lighter and easier to mount via a chest harness or helmet mount or similar. The best action cameras also tend to have better video specs, offering pristine 4K in high frame rates, which many waterproof compacts currently lack.