I don't know what island you're referring to, but there are a lot of claims out there like this - a lot of people claiming they speak the "Queen's English". Every claim I've heard, much like Eskimos and snow, turns out to be a myth:
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 :)