This mod restores the cut content, and more specifically the ability to use weapons underwater (R* originally planned to add a harpoon, information obtained from HeySlickThatsMe).
Instructions before adding the mod to the game: Replace the file called "weapons.meta" , using OpenIV to open the feature: GTA5/mods/update/update.rpf/common/data/ai.
My inspiration was the YouTube comments written by HeySlickThatsMe, TheNeoFelicity and rokkudayo.
What I did. Looked up "weaponflags" and then added "UsableUnderwater".
Be aware of the bugs that when your character gets stuck underwater, use the trainer to teleport to land. Based on the cut content with the harpoon weapon.