Ahoy, me hearties! Today is so-called International Talk Like a Pirate Day, a spoof holiday created in 1995 by a couple of friends (more information here).
Pirates all seem to sound the same in films; they all have the same accent, even though, in reality, Yorkshire pirates would have had Yorkshire accents, Cornish pirates Cornish accents etc. The reason that we all assume that pirates speak with a West Country accent is because of one actor, Robert Newton, whose portrayal of Long John Silver in a couple of films in the 1950s became the standard for future actors. Newton was born in Dorset, and exaggerated his own accent when playing the pirate. The West Country accent is rhotic, which means that the r is pronounced in words such as shiver, timbers, hearties (I do not pronounce it in my accent, nor is it heard in Received Pronunciation). That is why "aarrrgh" is a typical pirate sound.
Here's a ridiculous video on YouTube purporting to teach viewers how to speak Pirate.