[Capt. Debbie]

99% likely the ground with a dirty connection. Especially with two different signals both screwed up

Those gages read millivolts. A voltage drop across a shit ground connection will do that.

The whole boat electrical system has the same assume ground reference to measure voltage against. Whether millivolts or volts. And if there's a slight different is the ground at one point versus another the small signal stuff is F'd.

Ever hear of stray currents at a dock? That's exactly it. AC ground carried out to the boat, may not be exactly 0.0 VAC when compared due to a dirty ground carrying connection. Boats with chargers - the underwater running gear O/B's, outdrives and Prop shafts are all connect to the motor and the ground. Still no big deal. Separate system. Then connect shore power including battery charger and the underwter running gear has the dock AC power 0.0 VAC ( or not with degraded dock AC wiring).

So if it's 0.0VAC at the bulkhead and your boat's ground is 1.5 VAC ( for example) higher- you will be electricity in the water. Very high electrolysis and if high enough you can feel it in the water. If even higher you can be electrocuted.

A boat you're dealing with relatively low and DC voltages of 14vdc or signal grade- mvolts. So the ground connections everywhere should be the same. SO to test measure ground at Neg battery post and ANYWHERE else. The should all be 0.00 vdc. If not---- you have a shit ground connection somewhere.

I hope this helps?