You can't change the water of one place of the map. Is either all the map water, or nothing.
There's an action to set the water color under 'Enviroment'. You can change the water for only 1 player, depending on your event (It's usually "A unit enters region") or something. Just do this
Custom script: GetLocalPlayer() == GetTriggerUnit() then
*YOUR ACTIONS*
Custom script: endif
GetTriggerUnit() means "Triggering Unit". It depends on your event. Can be Triggering Player [GetTriggerPlayer()] or anything you want.
Be aware that the bad use of GetLocalPlayer() may cause desync (all players get disconected from the game). You cant use any kind of 'Wait' actions, nor set any variable value, do some cinematic stuff, and else. There are some tutoriasl about GetLocalPlayer() in hive and google