GtaGamer222 has made a mod that does pretty much as advertised: it covers Los Santos in water, so much so that only the very highest parts of the city and surrounding countryside have escaped the deluge.
Hereās a video by GtaGamer222 running around and piloting a sub:
https://www.youtube.com/watch?v=3yh05AUxz7c
While hereās a good one by MkElite thatās got a lot of first-person action, as well as a look at what happens when you change the weather conditions.
If this looks familiar, thatās because a very similar mod was made for the console version of the game. Not sure that one could spawn giant whales in downtown, though.
https://kotaku.com/you-can-sink-los-santos-with-gta-vs-game-changing-mod-1509412694
Time to add this to the gameās custom soundtrack: