Vicki, I think you want to have your cake and eat it here, and you know you can't do that!!

The basic problem is that the reflection from the water is brighter than both the water itself and the fish. So the blues in the reflection will always dominate the softer greens of the water. If you add the image with the reflection as a layer above the image of the fish, and then reduce the opacity of the top layer to let more of the fish image through, then as you've discovered you simply end up with neither one thing nor the other.

You might try the following, although I'm not guaranteeing it will look any good!

1) Create an image with two layers, one layer containing the reflection image, and one containing the polarised image.

2) Duplicate the layer containing the reflection image, and then order the layers so that the polarised image is the top layer, and the reflection layers are the background and duplicate.

3) Select the fish from the polarised image, and then feather this selection by, say, 10 pixels.

4) Use curves to boost the contrast of the fish selection.

5) Copy the selection, and paste it into the duplicated image with the reflections.

6) Delete the polarised layer, so you now have two identical layers, except that the top one has the fish from the polarised layer pasted into it.

7) Reduce the opacity of the top layer to your taste. As the layers are identical except where the fish is, you can then control how much of the reflections you allow to show "through" the fish, as it were.

I've taken the liberty of posting my "version" of this, but please bear in mind that the resolution is hopeless as I was working from screenshots of small parts of your original!

Steve