Smooth water [iPhone XR: 1sec exposure with ND filter in CameraPixels Pro & edited in Lightroom]: iPhoneography
Pro ND Filters for Your Phone - New PolarPro Iris Mobile Filter System
Amazon.com: Mccool Universal ND Filter Camera Lens with clip Kit Adjustable Fader Neutral Density (ND2 - ND400) Super Slim for mobile phone Kit for Iphone 6 Plus 5s 5, Samsung Galaxy S6
iPhone ND Filter - SANDMARC
SANDMARC Motion Variable ND Filter for iPhone Launched | CineD