First, for some reason I was visualizing the 60hp oil pump. I take the cover off of my 90 every 2 weeks to check or fill the oil so I should have known that the pump wasn't inside the tank. The rust on your pump is in the casing that surrounds the magnetic coil. You should be ok brushing it off and painting or coating it wit grease to prevent corrosion. There should be no water getting to that location. Take a look to see if there is any sign of a gasket leaking and spraying water there.
I had to do some research in the parts catalog to figure out why I thought the oil pump was in the tank. It was in the tank on the 04 - 09 models. We don't ever have to do much on the I-3 engines except service so I had never noticed the change in the tank and pump setup.