The International Maritime Bureau, a division of the International Chamber of Commerce, keeps up-to-the minute maps of global piracy, with linked data on the attacks. It's definitely worth checking out.
Above, the purple tags denote "suspicious vessels," the yellow "attempted attacks," and the red "actual attacks."
Parsing the data, I counted that of 45 attempted attacks in the Gulf of Aden, 7 succeeded; in the Indian Ocean off the coast of Somalia, of 31 attempts, 11 succeeded. This implies a pirate strike's more likely in the Gulf, and more likely to succeed in open waters.
Peter Pham takes a closer look at the technicalities of pirate attacks, and stopping them, today on FP's website.
(Hat tip: Global Dashboard)