I'm going to put an end to this myth: The "Giant Submersible Wreck" Easter Egg is most likely just an abandoned underwater research facility/lab. Comparison photo - the Aquarius Reef Base. :
Hidden Packages - GTA 5 Guide - IGN
10 GTA 5 Secrets That You Won't Believe - Thelma and Louise, Aliens, and Underwater Hatch | VG247