Think Mike is correct. There you are in fresh water (maybe not sweet), not the best electrolyte, so it can't be everyday galvanic corrosion, right? So it's the hot marina syndrome or your 'bonding' system has the motor in a loop. A loop that has a fault in it.
Looked real close at 338's 8/4 (2001) Yamaha and was astonished to find two sets of small zincing wires (what's the word: depotentializing?) in totally vulnerable positions in or near the water. Both connect moving to stationary parts at the base of the clamp and at the bottom 'hinge' of the leg. These wires could easily experience a little accident and break a circuit, if circuit is the correct term.
There is an exposed zinc on the motor at the clamp, and a larger square shaped zinc hidden under the cavitation plate right over the prop. You can't see this zinc (if you don't lift the OB) unless your boat is out of the water - or you are in the water. Wouldn't assume anything until you actually touched these places.
Heard that lanolin is an excellent corrosion inhibiter on props. Maybe you can smear it on your prop in the water or the whole unit in the water, warm it up first, the lanolin - or the prop. Not kidding about sheep grease, look it up!
You can over-zinc, over-power the anode effect of the zinc - ie have too many or have them too large, so they won't work properly. Was wondering from your description of your grounding plate if it could be creating a problem in a similar way? You know, something out of balance between the g. plate and the ob always in the water. Does your grounding plate show signs of acting as the cathode?
Maybe our Yamahas are primarily set up for salt water. The zinc (which is about 3/8' of an inch above the tips of the spinning prop blade on 338's) may not work as well in non-salt water. So you are not getting the anode/cathode reaction as intended. Wouldn't you suppose the problem is occuring when the boat is inactive, sitting in the marina?
The prop itself could be at fault - have impurities in the alloy or experiencing galvanic corrosion in the grain of the material. Maybe a bronze prop is the way to go. Painting the prop itself with zinc, or keeping it painted with 2-part urethane, maybe willie's 5200 method would work. Kidding aside, you could get the prop plated with a more inert metal like tin.
But lanolin is SO comforting natural, much less expensive, and it's good for your hands, too
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