As an amateur diver and holiday vlogging enthusiast, there is one useful piece of equipment that I decided to try with my GoPro a few years ago.
When filming underwater, you might notice that things tend to seem infinitely blue and washed out. This is due to the absence of red-light under water.
To remedy this I tried a filter on the front of my camera and the results were great. So if you're planning on doing some shooting underwater, consider a red filter of some sort to balance colors.