Just been playing around with the water again, and thanks to Nbt's explanation earlier, I now understand about the height of the water.
Basically, in the script the height is a 3 digit number, the 1st number being the level of the map, ie level 0,1,2,3,4,5 etc, the 3rd and 4th numbers being the percentage of the segment height eg 25, 50, 99% etc.
So, a water height of, say, 375, would mean the water is at level
3, taking up
75% of the height of a segment.
Sorry if I'm just repeating what others have said, but I like to get things clear in my own head as well!