Missing Letters
There are a few letters you should never pronounce. The first of them is "g". Drop all your "g"'s when you speak and you'll get words like "rowin'", "sailin'" and "fightin'". Dropping all of your "v"'s will get you words like "ne'er", "e'er" and "o'er".
Big, Bigger, Big Biggest!
Pirates are dramatic, and their speech is doubly so. Pirates never speak of "a big ship", they call it a "great, grand ship!" They never say never, they say "No nay ne'er!" Double up on all your adjectives and you'll be bountifully bombastic with your phrasing.
De-Conjugation
The conjugation is a rather modern invention, one that sailors always seem to be forgetting. Take the verb "to be" for example. Instead of saying "I am", sailors say, "I be". Instead of saying "You are", sailors say, "You be". Instead of saying, "They are", sailors say, "They be". Makes things a lot simpler, doesn't it?