Just to follow up on this. Pulled the boat out. All anodes on the motors are aluminum alloy anodes, the hull weld-ons are zinc. I scrubbed them all with a SS brush and hit the rivers. Voltage back to normal. Grabbed a piece of SS downrigger cable and hung it over the edge and also checked my voltage from the hull to the water. Both right back where I wanted them.
I'm not 100% sure what was going on with it in the lake, but I did get some bubbling under the small painted areas of the hull too. The only thing I could think of was that there is an aluminum boat lift about 20' away. Maybe the combination of the treatment, that aluminum structure and an extended mooring in freshwater did it. I do know I didn't touch any wiring in the whole boat though. Could also be all the chemicals from people's fertilizers running off into the lake?? Who knows.