Look what the ocean threw up.

I like to think it was lost in a canoe accident.
Been lots of those as of late?😉

First realize you will never get it back to "Perfect new". But can make it usable and decent looking.
Buy yourself some "Evapo Rust". You just leave stuff soaking in it and it will remove all oxidation, including Blueing. Best part. You can reuse it several times, on other projects before it looses effectiveness.

First, wire wheel, or brush the caked on stuff off of it. Preferably with a brass brush. Then use the "Evapo Rust" according to directions. Then a good scrub with soap and hot water.followed by a very quick drying.
You can fill the pits with high heat JB weld then sand them smooth.

After that use black header paint with ceramic and cure in the oven a bit.
Or you can buy one of the special spray on gun coatings which is pretty much the same stuff with a higher price tag.

The inside of the barrel can be cleaned up by packing it with a rag soaked in Evapo Rust over night, then just let it soak. The stuff only removes oxides like bluing and rust.
If the barrel has some pitting inside, often it can be brought back to usable and fairly accurate.
By just cleaning the heck out of it. Then shooting lead .22 lr out of it till it straightens up. The lead will fill the pits. After that never clean the bore, just run a rag with regular gun oil down it after use, to pull out the powder residue.

OSPHO IS GOOD STUFF. I have used it as a rust converter.It works real well.

Kinda turns rust to a bluing. Used it in body work for years up here in the Rust belt.

The reason I suggested "Evapo Rust" is being in salt water, might be a good Idea to remove all the rust to bare steel, and not to convert the last little bit that is hanging on.
However I believe Ospho would work.
I just have always used "Evapo Rust" in instances where I absolutely wanted bare perfectly clean metal and not rust conversion. And did soaking. At least for the first step.
Plus I have had a gallon of Evapo Rust that has lasted me hundreds of small projects, and is now just getting loaded up to where it takes a while to work.

Duracoat is a good viable option too, I believe they even make a putty like pit filler.
I just like the header paint because once rubbed down lightly with a bit of 000 and oiled it looks just like some of the old K type mouser finishes.
all kinds of options out there.
You defiantly aren't going to make it any worse, so have fun with it! Use it as a learning experience.

here is a Glock 26 that went under about 8feet of seawater during Hurricane Irma and wasn't found for a couple months. I Duracoated it but the pits on the slide are so deep that they still show.