You identify an ESC intended for brushless motors and then state you're using a brushed motor with it. Bozo No No! You need to read up on brushed and brushless and the ESCs that support them.

One of the things most people want in a boat is reverse. Not all ESCs do this and in particular not all brushless ESCs. So you need to decide what type motor you're going to use and proceed accordingly. Nothing wrong with using brushed motors in your application. More options IMHO for boating use.