Open your Threshold dialogue box : In the Threshold dialogue box, pull the little slider at the bottom all the way to the left. See the remaining bits of your photo? Those are the darkest bits. .Get the Color sampler tool ( make sure your sample size is 3 by 3 average ) and place a mark on the darkest spot
Do the same on the lightest spot (pull the little slider at the bottom all the way to the right ) and place a mark
Now just drag the Threshold adjustment layer on trash
You will able to see the 2 mark as 1 and 2
Now Choose Layer > New Adjustment Layer and choose Curves
(Use Curves for color cast and Levels for contrast )
On the Curves Layer get the black eye eyedropper and sample the #1 mark ( this is your Darkest spot on the picture)
Do the same on the lightest spot, get the eyedropper and sample the #2 mark ( this is your lightest spot on the picture)
Now you will see all the color cast is gone
We have to do one more thing before we finish here
We have to find the natural gray spot on the picture this way we can balance the picture correctly
Then if we know are darkest ,lightest, and the natural gray were they call it 50% gray in the picture we will have a prefect white balance to start with
Create a new Layer
Go to Edit/Fill/ and under contents choose 50% gray /then OK
Now under blending modes choose difference and you will see a deferent look in your picture
Go to Layer > New Adjustment Layer and choose Threshold adjustment layer
Pull the little slider at the bottom all the way to the left and get the Color sampler tool ( make sure your sample size is 3 by 3 average ) and place a mark on the darkest spot again #3
Now just drag the Threshold adjustment layer on trash
Double click on the Curves layer and this time we get the middle eyedropper and sample the #3 mark
This is going to give you the 50% gray on your picture
Wow!!! that was a lot for me to type
If you don 't loose your cast in the water we will move to the next step ...
I do this to all my pictures and I call the Curves layer global
It's always's nice to start with a picture were you know it is not color cast on it
It's not going to give you a 100% color cast free picture but it is going to get you very close to it
I hope this will help
Thanks E-man