I found that the best time to go for this cheevo is when you're searching for Submarine parts. Purchase the Sonar Docks at Paleto Cove and speak to Abigail with Michael (Strangers and Freaks mission). You will then recieve a dinghy to drive around in to search for Submarine Parts. You are bound to run into a shark sooner rather than later as you searchs the depths of the ocean. I ran into one just before finding the 4th Submarine part.