I found the best thing to do for this trophy is use a helicopter, you HAVE to explore the shoreline. It defiantly is easier at the start of the game as well, I know it can be tedious and it does take a little while but it does unlock it
I used this method for the xbox 360 version and ps3 version of the game too, If you have any problems send a message and I'll do my best to help you :)