This looks like it might work. However, I am not sure how it will hold up to saltwater.
http://www.ebay.com/itm/Hall-effect-G3-4-Flow-Water-Sensor-Meter-Digital-LCD-Display-control-/161142903824
There is this one too.
http://www.ebay.com/itm/Orbit-Water-Flow-Meter-New-/351883012037