EDIT: Here are some native English speakers: http://www.youtube.com/watch?v=f1TJnDG61_Y Can you understand them? I sure can't! :)
(The most incomprehensible language I've ever heard from a native speaker personally wasn't Glasgwegian, but rather two 60+ folks demonstrating Black Country English.)
They speak English, but I just could not understand anything the locals said. The dialect/culture is called Gullah: http://en.wikipedia.org/wiki/Gullah
The "18th century" thing stuck in my mind from some media piece, and now that you mention it, does sound rather obviously like a myth. But the place exists—I think it was probably Tangier Island [1], and the accent there is indeed archaic:
http://www.youtube.com/watch?v=AIZgw09CG9E#t=0m33s
Some of that sounds like a Monty Python sketch! It's definitely an example of one native English speaker (me) finding another (them) hard to understand.
I also second the commenter who brought up the Glaswegian accent. I love how it sounds but damned if I can make out half of what they're saying.
Maybe you're just gifted with dialects :)
How well do they integrate with modern English speakers when they leave the island?