If you have a GoPro camera and would like to take images like the one seen above, you want a dome lens. Though if you’re very careful, you can accomplish this type of shot with a normal GoPro, but you have to place camera very accurately where the water meets the air, a difficult proposition given the waves that nearly always exist in a body of water. A dome lens gives you several inches of positional leeway, and you can build your own following the instructions after the “read more” link!
On the other hand, there are several “BIY” options on Amazon if you’d like to check that out instead. Though I can’t vouch for the quality of all of them, some of the prices aren’t too far from the DIY option.