I'll offer some anecdotal but real life experience.
We've run our boat for 8 years semi-bonded. That is, we have a galvanic isolator
on the AC system and the boat has a hull
zinc. Most, but not all thru-hulls are bonded to the hull
zinc. The shaft (hence the prop) Is not bonded (shaft brush failed long ago), but there are of course zincs on both the shaft and prop. When in a marina, I generally hang a grounded (to the bonding system) zinc over the side.
To date, I see NO difference in corrosion between the bonded and unbonded thru-hulls. My prop and shaft are fine, and my zinc generally last several months.
The only time that I have problems have been in marinas
and moored to a hot boat. In one instance I noticed that my hanging zinc nearly disappeared in less than a month. Problem went away when I moved slips. Shortly after, I added the galvanic isolator
and have had no serious issues, but one other time, when near a suspect boat I did notice some additional loss of my hanging zinc.
Finally, I do make it a practice to disconnect from marina power supply when I do not need the power. I have sufficient solar
to provide most of our needs when at a slip.