No Need To Be Koi About It - Use A Polariser
Except when the skylight blocks out the surface of the water. Then you see whatever is bouncing off that surface. It is the same with shop windows and other shiny surfaces.
Answer is, and always has been, to use a polarising filter on your lens. The older types were known as linear polarisers and did a very good job of seeing down into water surfaces. The newer types are called circular polarisers and may be a little less effective in actually penetrating the surface, but do give a more accurate light meter reading. The Circ Pol's are the ones most often supplied for modern digital cameras.
We've got 'em from Kenko, Hoya, Promaster, and B+W. They are not as cheap as UV filters - never could be - but they are a pretty essential tool for landscape and marine photographers. Useful, too for correcting colours under trees in open sunshine - you can lose the blue fill from Western Australian skies. particularly recommended for bridal work in these circumstances.
Studio? Well there are times when you need to see into things and these can help. You can also use them with sheets of polarising film and studio lights if you are going to copy glossy or textured flat art work. a little more complex than just twirling it in front of the lens when you are out at the beach, but essential to capture the true colours of some canvases.
Note for newbies: Either put a UV on the lens or a Polariser. Not together. Too many glass surfaces, and sometimes you start to unscrew the UV when you think you are turning the Circ Pol.