Increasing the decay distance makes the water clearer for a further distance before it becomes the colour of the decay tint, it's measured in metres. Your lake and heights are of such a high scale(i.e. 3000m radius, when it looks to be a very close to the ground scene. It isn't though in terms of scale. Test this by changing the 'y' value of the lake transform by ten metres or so, it barely looks like it's moved).
Because of this large overall scale, low levels of transparency decay, like you had before, have little to no effect that you can see. I'd still probably raise it a bit higher than I've changed it to. You feel like you're 2-3m above the surface, you'd see more through the water at that height, but you're actually much higher, so large values of decay are needed to make you believe you are still close-up to the surface.