Using the raw file, in photoshop
select channels
uncheck the top channel that looks like blocks
then highlight the bottom channel with the smooth colors
this is the one to edit.
to get the right color use the eyedropper
go to the area that is on the border of the water
it will have 3 of the same number, example 58 58 58
then move the slider down 3 or 4 to show 54 54 54 all three must be same number
hit ok and use this to paint the water area.
A quick note. all of the maps range from 0 0 0 to 255 255 255
0 0 0 is bedrock dead bottom, 255 255 255 it hard limit of the sky
when all three numbers match it is always shades of gray from black to white
To do it more naturally you would need to go to the Paint bucket
tool, and switch it to the gradient tool.
the pic should help
You can do it, next you will start full teraforming,