Gradient’s parameters:

Select the next brush on a new layer

that would have the black color, just to represent on the picture’s lowest part the water – plants.

Create a new layer and select a brush of medium size, having the opacity of
30% to dark out the lowest part of the picture. Choose the next color:
1D4571.

Set the
Hard Light on the layers.
Take the next instrument

to represent an air bubble.