Using Sculling Coach requires the phone securely mounted in the boat for best results. There are several waterproof case/mounting systems that work well - the one displayed is made by Armour-X. Other good systems include ProShot and LifeProof.
Since ScullingCoach is intended to give constant feedback, the phone setting for auto-lock needs to be set to never. If the screen turns dark during a row, it is difficult/impossible to reset the phone when your hands are on the oars.