Batteries not staying charged

[Yukon Craig]

I have a Yamaha F115 four stroke. I was out recently and both my batteries went dead while *****ing. I checked all the grounds, recharged the batteries and only get a 9 volt reading when testing. I suspect it is my regulator or stator but I don't know how to to check either. The red and black wires from the regulator plug give me a 9v reading as well. How do I check the other three white wires and what might I be looking for. Thanks.

Yukon Craig

[boats.net]

Have battery load tested and check battery cables and cable connections are clean and tight. Connect voltmeter to battery and monitor voltage, when engine is running above idle speed battery voltage should slowly increase if alternator system is working ok.