2004 VX225 2 stroke OX66 Voltage dropping??

Need help!!! After charging batteries(new optimas) and beginning my day of fishing, my sonar at the steering wheel has a voltage meter and indicates all is well (12volts with NO motor running and 14volts when running) but as the day goes on it drops and drops quickly and running the engine only holds the voltage steady. The voltage will drop as low as 9.9 to 10 volts causing my lights to be dim and my tach guage not to function. Is this a stater or rectifier problem? Note that this was going on with my old batteries as well.

[boats.net]

Have the battery load tested, check charge voltage at battery when running. It could be battery cables, regulator or lighting coil.