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. :
Image 6 - Secret Underwater Base Expansion [.NET] mod for Grand Theft Auto V - Mod DB
Underwater Base - GTA5-Mods.com
Underwater Base - GTA5-Mods.com
Underwater Base - GTA5-Mods.com
Iceberg Submarine Base by Hideyoshi on DeviantArt
The SECRET GTA 5 Underwater Valley You Probably Didn't Know About! - YouTube