You can cycle Barnacle Buster through there. What you would need to do is get a 5 gallon bucket. mix the barnacle buster with water as per the instructions. Remove the discharge hose from either the AC unit or from the through hull. Connect a hose to the AC unit or discharge hose (depending on which you removed). Then run teh host into the gallon bucket. Disconnect hose between Sea strainer and AC pump. Connect a hose to the AC pump and stink that in the bucket. Now, turn on the pump. It will draw the water and barnacle buster from the bucket, into the pump up to the AC, then out hte discharge hose and back to the bucket again to make a closed circuit. You'll need to make sure you know how much fluid the system holds, including the hoses so that you can add the correct amount of barnacle buster. One way is to count teh exact number of gallons that fill the bucket. As the level of water in the bucket goes down, continue adding whole gallons until the bucket is close to full and fully recycling. now use the amount of gallons you counted to determine how much barnacle buster you need to add.
You'll need some pH test strips to periodically test the water. Once the level exceeds a certain amount, then flush all the water and start over again. This neds to be recylcing for a couple of hours. You can do the same thing with an external pump, such as a small bilge pump or very small sump pump.
Check out hte Barnacle Buster website, I believe they have a very expensive kit to do this with, though I think you can build your own for much cheaper. They also have the pH strips as well.
I would bet growth on the internal coils is what is actually causing your problem. If not, don't discount simple corrosion on the breaker itself.