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
Well it's not the actual water tint, but there is a simple way of making water different colours in different parts of the map.
First of you all you must download UTM (Ultimate Terraining Map) Then copy-paste the water model into your own map, change its tint values, and simply make these doodads wherever is needed. I recommend you either use raise to override current water (may require 2 or even 5 water doodads on top of each other) or use the lower tool without making any water, place pathing blockers all over the lowered part of the terrain and make this section full of water doodads.
Other then that Spartipilo's suggestion is the only other one that is really feasible and I know of. Hope this helped.