I remember when GTA IV first came out, I was surprised I could swim in Liberty City. I fell off to the ocean, I was shocked that Niko could swim.I guess fool me for having GTA:SA as my first GTA game since it spoilt me with all the open space areas, Bigfoot mysteries and even entering water safely
GTA V has a surprisingly developed underwater world. Marine life, submarines, triathlons, even swimming stats. One side mission really lets you explore the underwater world.