Thanks for getting back to me Zypher69, like you I have found a soft white / black brush to be best.
Haakoo, here's an example I have made:
I want this final result made from two images:
Big wave surfer by Duncan Rawlinson available from Flickr under Attribution-NonCommercial 2.0 Generic (CC BY-NC 2.0)
Playing a fish on the Boat Pool, Kinermony by Des Colhoun available from geograph.org.uk under Attribution-ShareAlike 2.0 Generic (CC BY-SA 2.0)
In Photoshop, you would import both images and add a mask to the wave. You can blend the images together using a variety of tools - Brush / gradient / Fill.
The tools are destructive to the mask - it's just a mask, it doesn't have any important data on it.
The finished mask looks something like this:
As you can see the mask is complicated, its not a simple gradient.
In my workflow in Photoshop, I primarily use the gradient tool due to its speed. The ability to change radius by clicking and dragging means I don't need to keep changing brush size.
In AP, I am restricted to using the paint brush, and whilst I can get a similar result, its not as quick.
If I try and use the gradient tool on the mask, it wipes what I have already made using the paint brush.
If I use the gradient tool again on top of the first gradient, it wipes the previous gradient.
No one is saying the gradient tool is hard to use, we are saying a single gradient per layers slows down or restricts development.