Digital Dude here. What do to with the iPhone when you are shooting water? As you can't control the shutter speed or the ISO, the answer is.... not much. Most shots turn out like the one below with a speed not quite fast enough to freeze the action and not quite slow enough to make a nice blur out of it.
There are no neutral density filters for the iPhone to help slow the shutter speed, but the is an app called SlowShutter. It doesn't reduce the ISO but takes many multiple exposure over a given period of time. If the subject moves or if you move the camera, the result is a delightful blurring effect. So, in the middle of the day, without any filters, you can turn the above shot into the one below. Frankly, unbelievable!
One more thing. As my dear wife keeps putting up her ToonPaint pixs --- and as I found the app first (no competition here) -- I had to post one too. Meet "Darth Kitty"! Straight shot of our cat; mirrored the image in Symmetry; cartooned it and painted the eyes in ToonPaint; brought back his color in DXP.
You'll be seeing him in the next Star Wars movie.