https://www.techradar.com/news/polarizer-and-solid-nd-combine-into-one-single-filter-perfect-for-waterfalls

Singh-Ray filters has teamed up with photography Bryan Hansel to launch the The Bryan Hansel Waterfall Polarizer filter, which combines a polarizer and neutral density (ND) filter into one single filter. 

Specifically designed to capture waterfalls and river rapids, combining the two filters into one means there's no longer need to stack multiple filters, which means that potential hurdles from stacking filters, such as light-leakage and vignetting, are avoided.