There are basically two different approaches you can take to a topic like this, either focusing on the adventures of a single pirate or sailor, for example, or finding four different characters whose stories you want to tell. If you go with the four-character option, then you have to decide just which four characters you will choose and what each story will contribute to your project overall!
Wikipedia Pirates. Wikipedia is definitely your best source - check out the main Piracy article, along with this ENORMOUS list of pirates organized by historical period; there is also a this list of women pirates. Each of the major pirates whom you see listed there has an article of their own in turn where you can find information both about the pirate's history and also about the legends associated with them.
Wikipedia Sailors. Instead of historical pirates, you might instead want to work on the famous sailors of mythology, and once again Wikipedia is a great place to start. You can find detailed articles about the legendary voyages of Greek sailors like Odysseus or Jason, a Middle Eastern hero like Sindbad the Sailor (whose voyages provide the reading for Week 9 in class), or the great Celtic sailor known as Saint Brendan.
Online Books. You might also want to consult some online books about sailors and pirates at Google Books and other online book archives.