From what I can see of the bottle and lead foil seal, it has not been submerged in seawater for very long if at all... Sounds fishy to me... Not to mention that relatively low alcohol products like beer do not store well at all over very many years even when pasteurized pre-bottling. My mother once brought me an old bottle of Gilbeys Gin that she had bought at an estate sale. Seems the gentleman who had passed away had some kind of obsession about buying booze and they discovered the basement of the home was filled with unopened bottles upon his demise. When unsealed and opened, it smelled just like the dug moldy basement it had been found in.

 

Divers find 162-year-old Guinness in shipwreck off the English coast
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I just read that Guinness was started in 1759 in Ireland. The same reference quotes dark beer started to be made near the end of the 18th century which fits with the estimated timeline of the bottle.

I just realized I haven't had a Guinness Stout in years now.