It's hard to tell from the photos whether that's corrosion or bio-fouling. But if you're in saltwater you should consider tracking down and fixing the cause of the corrosion now if you are on the hard. Corrosion due to dissimilar metals occurs much faster in warm saltwater vs. cold fresh water.
For a cost reference, 4 years ago I had my 32' hull sandblasted to "bright white" ($600) and immediately etched it with one coat of vinylux ($65) and put on 5 coats of 2-part epoxy primer barrier coat ($260) before 2 coats of antifouling paint for aluminum ($300). Prices today are probably 20%-25% higher.
Peter
1981 32' sedan bridge
twin Chrysler 360 cu. in. 250 hp engines
Raw water cooled
Nimbus II
Home port: New Buffalo, MI