Mastercaster,
Checking your voltage is easy to do. Use a digital volt meter, put your boat in the water head out away from other boats and drop your downrigger ball down about 10 feet. Turn all of your electronics off in the boat and measure with the negative meter lead on the Neg Terminal of your battery and the positive lead to your downrigger cable. You should read somewhere between .5 and 1.0 vdc. If not you need to check you zincs. Now turn on all of your electronics one item at a time. You shouldn't see much of a change as you turn these items on. If you do you probably have a grounding problem with a particular item. When all of your electronics are on, you should still be between .5 and 1.0 vdc.

Make sense?