Stormin
I don't have a lot of experience with all this, as many know I have only owned downriggers for a few weeks
Mine had homemade black boxes/ion control on them that were external and did excactly what you are talking about Stormin but the voltage on the cable was closer to 12v and the darn things were wired into the motor circuit on the 'rigger so the only time the whole contraption would work was while the 'rigger was going up/down. I disabled and removed the home made stuff after talking to the service guys at canon (very helpful, even faxed me wiring schematics ) and he was flat out alarmed that there was any voltage fluctuation more than .1 v or so at the cable at all.
I have not had an opportunity to measure the voltage on mine in salt at all yet, or any other tests that you mentiond yet. Nor have I fished with any other downrigger in my life, so I am likely the least qualified person here to ask any question at all or come to any hard conclusions, it just would seem to me that since Jerry is basically running two black boxes on the same 'rigger wire( positive ion control and a black box) they are competeing with one another espescialy since the voltage fluctuation is so low ( several hundreths of a volt not 7 to 9 volts. In addition to all the recomended tests Stormin mentioned check out this link if you haven't yet. They have posted their guide on the net for all to read and it goes even deeper into depth on the troubleshooting Stormin was talking about http://www.protroll.com/blkbox.html
Taking all of the above into account it would seem logical to try using each unit without the other and see what the voltage is. The only problem might be in turning off the P.I.C. at the 'rigger but I would think if you unplugged it at the power source Jerry, and killed all power to the rigger except what the BB is doing you might come up with a nice steady voltage. I agree with Easy and am very curios as to why you do not use the factory P.I.C. on the 'rigger to get the voltage where you want it? I am not trying to seem like a know it all, I have no experience with the P.I.C. from canon( or anything else concerning downriggers) And I definitly do not want to second guess storminN I am just trying to follow a logical train of thought here and help out... Of course I am probably a day late and a $ short and you have already got it fingered out...