The GoPro is an excellent camera, but your results will vary depending on water visibility and conditions. However, the camera does not do miracles just by turning it on. Taking a little time to practice how to keep your shots as steady as possible would do wonders for your videos. You can practice in a pool holding the camera in different ways to see what yields the bests results, or shooting the same subject with the different settings and reviewing the results may help you understand how to use the camera in different situations.
The reason for adding a red filter is to bring back the red spectrum in the sun light being filtered by the water. The deeper you go the more sunlight gets filtered. And not all Red filters are created equally. I have Red filters for shallow, clear blue water and a different red filter for deep, clear blue water.
So if you're snorkeling in really clear water (basically you are on the surface) light is not being filtered, therefor there's no need to add the filter. However, if you dive the very same site and are at lets say 10 feet shooting towards the bottom at 20 feet the red filter will make such a marked difference in the video. Now if you instead begin to shoot at 10 feet and turn your camera with the red filter installed towards the surface you will see odd orange color looking video.
I hope that helps