I may be too late but what you want is a blending mode, and I believe some have the same name either in krita, gimp or photoshop!

You need a blending mode on the reflection layer that shows the lighter values and ignore the darker ones.

Then you paint the seabed or whatever is underneath and simply apply one of the lightening blending modes (and there are some besides Screen that do the maths a little different so you have to experiment the best one for your case) available at you software of choice on the reflection layer. That is going to show only the clear pixels of the layer if they are clearer than the bottom and ignore them if they are darker, or some variation/percentage of that depending on the coding used in the specific blending mode.

Remember opacity and fill sliders in Photoshop's case may have different results depending on the blending mode used, that may be true to krita as well but I'm not sure.

I may be too late but what you want is a blending mode, and I believe some have the same name either in Krita, GIMP or Photoshop!

You need a blending mode on the reflection layer that shows the lighter values and ignore the darker ones.

Then you paint the seabed or whatever is underneath, and simply apply one of the lightening blending modes (and there are some besides Screen that do the maths a little different, so you have to experiment the best one for your case) available at your software of choice on the reflection layer.

That is going to show only the clear pixels of the layer if they are clearer than the bottom and ignore them if they are darker, or some variation/percentage of that depending on the coding used in the specific blending mode.

Remember opacity and fill sliders in Photoshop's case may have different results depending on the blending mode used, that may be true to Krita as well, but I'm not sure.