instructions

Instructions
***********WATER*************
(For the mission maker version this step is not necessary, it is nice even without ... you decide ...)
- Before opening the game, run OpenIV and go to edit mode, Navigate to common.rpf/data/level/gta5
BACKUP water.xml in case something went wrong. 
And then replace it with the one included in the archive. 

**********XML***************
Then open game and load the underwater_base file with the editor.
You need Map Editor by Guadmaz https://it.gta5-mods.com/scripts/map-editor,
copy the file underwater_base.xml in your GTA root
F7 -> load -> xml -> underwater_base

**********YMAP****************
run OpenIV and go to \mods\update\x64\dlcpacks\CustomMaps\dlc.rpf\x64\levels\gta5\_citye\maps\custom_maps.rpf 
activate Edit mod
drag and drop the file underwater_base.ymap
closa all, open GTA and play




***********************Mission Maker version************************

(This is needed for a new mission you can find here: https://gta5newmissions.altervista.org/)

You can play with the xml version, just load it before start the mission, and enjoy.
copy the file underwater_base_mission_maker.xml in your GTA root
F7 -> load -> xml -> underwater_base_mission_maker

or you can use the YMAP version
run OpenIV and go to \mods\update\x64\dlcpacks\CustomMaps\dlc.rpf\x64\levels\gta5\_citye\maps\custom_maps.rpf 
activate Edit mod
drag and drop the file underwater_base_mission_maker.ymap
closa all, open GTA and play






If the ymap version doesn't work, it's probably because the latest updates overwrote your dlclist.xml file. 
You have to open it in OpenIV, edit it and add <Item>dlcpacks:/custom_maps/</Item>, and restart the game.
I don't know if that's normal, but mine lately does it from time to time.