If you want to take photos through surface glare into water, or just want to cut down surface glare, you are going to need a polarized filter. This otherwise beautiful video doesn’t demonstrate exactly that perfectly, so I added a comparison photo that demonstrates it better. By the way, you can get the same effect by wearing sunglasses with polarization, which might explain why so many swimming coaches use them. Via PetaPixel.
Polarizing Filters for Photo and Video from Olivia Speranza on Vimeo.