Hi Greylock,
I too have a V/P I/O. I keep the boat in the water in a 60 slip marina from May to Nov. Shore power is available, several larger boats are plugged in. My V/P is a 1993 D/P. Drive is original, motor repowered in 04. I have zincs on the prop shaft and transom shield. New zincs are installed at the beginning of each season. By seasons end, the prop shaft zinc is always worn the most, well beyond 50%, probably close to 80%, the transom shield zinc less than 50%. My drive has no galvanic effects. The aluminum is smooth as new - no pits. I am pleased with the job the zincs are doing. I think it is important to note the fact that the prop zinc wears the most. I would attribute this to the proximity to stainless prop shaft. Being the least noble metal in that area the prop zinc is sacrificed. I purposely have stayed with Aluminum props instead of SS, for just that reason. I venture that a set of SS props would accelerate the prop shaft zinc wear to the point it might have nothing left come Nov. Also - when shopping for zincs, there are Zincs made of Zinc, and "Zincs" made of another alloy. The other alloy "zincs" are cheaper and they do wear faster than Zincs made of Zincs.
Also - keep your drive all the way down when docked. The bellows will remain compressed and no barnacles will grow in the creases. I change the bellows every other year, as I pull the drive for a full service every other year.
Happy and safe boating in the new year.
Oceanbean
93 Explorer 245 V/P DP