Towards the back end on the bottom to there is "pitting" for lack of a better description. When I got this boat the bottom was absolutely filthy. Used muriatic acid to clean it. It got most of it off except for a film that buffs off with some work. Anyhow I don't know what to do about the "pitting" that is there. It's deep enough to easily feel with fingers.

Anyone ever seen this before? Will it hurt anything if left alone besides maybe scrubbing of a mph or two?

Cant tell too good from the pic, but it looks like what barnacle growth leaves behind when removed/cleaned up from bare fiberglass?

Yeah , it is kind of hard to tell from that pic , but when you say the bottom was filthy was it filthy from stains and calcium deposits/oils in the water or actual marine growth from being left in the water ?

I have seen gelcoat not only on hull bottoms but in interior surfaces such as c0ckpit soles suffer from a sort of pox like failure that results in thousands of fine 'chips' breaking out that do not go completely through and under the gelcoat like osmotic blisters. This seems to be more of a cosmetic failure. It could be that that is what's going on there.
Maybe some better photos wold help ?

It's the hardest thing to get a decent picture of it. I noticed it's only on one side of the keel. Not sure if I should just leave it alone. Sand it flat. Buff it.

My bottom has a little pitting where it sits on the trailer with the wet bunks. I don't remember what they are called but they are little pockets. They are so small in minor my boat survey inspector said do nothing but watch them just in case

salt or fresh water? Had this on a cobalt when I was growing up. Ended up being defect in the gelcoat. But, we ran fresh water only.

To my knowledge it's always been in freshwater. But there's been three previous owners and I only met one of them. Beings he was from southern Ohio I have my doubts it was ever a salt water boat.

Mine was a fresh water 1988 boat kept on a trailer and to add detail to my response above...the survey said: "Early stage osmotic blistering noted (small chemical dots) ---- These are considered cosmetic in nature."