I'll say it's clean and very well struck for being a 1738 coin. This is probably the best one I've ever seen. Probably too good.
It looks OK to me on the authenticity front (though I do not profess any specialist expertise). It is of course heavily corroded, and almost certainly was sea-salvaged from a shipwreck. If the wreck provenance was known, that would add to the value a bit, though the corrosion detracts from the value significantly. I have one in my "Eclectic Box" collection: Mexico (Spanish Colonial): silver 8 reales ("Pillar Dollar"), 1761-Mo-MM, Mexico City mint PCGS AU50; population 4 with 13 higher as of 11/21/2018.
The correct weight should be 27.0674 g. I suppose the discrepancy of your coin being slightly underweight could be explained by the corrosion, but I don't know for sure. The edge looks correct for the type. The corrosion makes it really tough to confirm as authentic for me, though I do think it probably is. (Take that opinion with the proverbial grain of salt, though.)
Plenty of coins of that era were well struck. Switzerland (Zurich): silver "city view" 1/2-thaler (1 gulden of 36 schillings), 1739 PCGS AU58; population 1- the only example certified by PCGS as of 11/21/2018.
With my collection habits, and pocketbook, I'm not used to seeing such well struck "population 1 " coins. lol
I would have never guessed that to be a shipwreck coin. Most that I've seen are still somewhat encrusted or otherwise damaged.
True, the norm for an 18th century one sees on a smaller budget (believe me, I know all about that, too) is much more heavily worn.
Once you remove the encrustation, you're left with surfaces like the OP shows. This is heavily damaged by seawater corrosion. It is called "shipwreck effect", and some people like it. If you have provenance for which shipwreck it came from, people will pay a premium for it. Otherwise, its just damaged.
Damaged coins that appear to be from shipwrecks are very hard to authenticate. Without a certificate of authenticity from the salvager or conservation company it could just as easily be a recent forgery soaked in acid. At least get a reading on silver content.
