Aside from Arrr!, which is essentially a glorified grunt, nothing brings to mind piratical speak like the words Ahoy! and Avast! (both of which have to, for some reason, have an exclamation point after them). Ahoy and avast as they have evolved are, in fact, the quintessential nautical words used exclusively in their appropriate context by seamen of all sorts.
Ahoy was originally two words, or so it seems. Ho! or Hay! was an exclamation meaning Stop! used by our seafaring ancestors from Medieval times and apparently deriving from Old German and Dutch, neither of which were too different from Saxon English. Hoay! or Hoy! was a word added to an exclamation for purposes of drawing attention during the same era as in: "Foremast top, hoay!"
The two words seem to have gotten together some time in the 15th century to become Ahoy! which began to be used as a hail. "The ship ahoy!" was (and still is to some degree) a common salute over the rail from one ship to another upon meeting at sea.
Avast has a slightly less defined etymology but it's meaning is unequivocal: Stop! Some dictionaries give the origin of the word as purely English and say that it derives from the shortening for convenience of "have fast" with have meaning "hold". In fact, it appears that avast is the English corruption of the Italian word basta - enough!
The use of avast became widespread by English mariners in the 17th century, when Britain had established naval bases in the Mediterranean. Here the Royal Navy would have heard Italian spoken regularly, and sailors are notorious for adapting their language to the local lingua franca. Avast is most frequently used in it's shortened form and in a battle or other situation where brevity is imperative: "Vast firing!" or "Vast that!" for instance.
So there you are, Brethren. Sprinkle a few ahoys and vasts into your speech, and sound salty without even trying! Salute to you. I'm for a mug o' grog.