Let me start off by saying that personally I think the Minelab Equinox 800 is a fantastic bit of kit. It does just about anything you could ask of a metal detector and is one of the most versatile machines out there right now.
However there does seem to be an issue when used in the water. Some have had seal failure resulting in several issues including very low volume output.
There is though a way to fix the Equinox yourself and in this video Calabash Digger explains what the problem is and how to fix it.