The main tool for this is a circular polarizing filter (CPL). Rotate it while looking through the viewfinder or at the screen until surface glare is reduced and the underwater details become visible. A CPL can also be used creatively to increase or decrease reflections on water.

Angle matters too: reflections are stronger at some viewing angles, so changing your camera position can help. Polarized light conditions and the direction of the sun also affect how well the filter works.

For a scene with dark water and bright snowy mountains, exposure can be difficult because of the high contrast. You may need to compensate exposure carefully, and in some cases use manual settings if the camera’s auto exposure is fooled by the filters. One answer also suggests adding a neutral density filter in some situations to help control exposure.

Use a circular polarizer rather than an older linear polarizer, since linear types can interfere with metering and autofocus on modern cameras.