Several databases will provide you with information about how many shark attacks have occurred.
Some of these sources provide statistics back to as early as the 1500s, with most offering data from at least the past 30-50 years.
Learn as much as you need to know about sharks before you come to Costa Rica.
One interactive directory has mapping tools that reveal where the unprovoked attacks have occurred.
Another database that might help is the Shark Attack Data site.