The first thing you want to do is purchase as many of the $500 maps of collectibles that you can. This way you have a general location of where you need to be for the relics.
The relics are scattered over both islands and once you collect one, you will forever have it. So don't worry if you take a deep dive in the ocean and die right after you collect it. Once you pick it up, you don't have to worry about going back and picking it up again.
Relics usually are hidden in caves and some are hidden in wreckage in the water. There are also a few relics that are just laying on the ocean floor but require you to dive fairly deep. I may be wrong but I believe there is a total of 120 relics. (If I am wrong, please let me know.) So thankfully you only need 60 to unlock the trophy.