How do I remove large scale variations in brightness in a JPEG image?

I have an oblique image over looking the sea surface. My plan is to mask areas of land and georectify the image to look at the surface expressions of the flow. I can mask the land and georectify the image fine, but it's hard to observe smaller scale variations on the surface of the water as they are hidden by the large scale variations in brightness - in particular the water appearing brighter towards the horizon.

I want to filter out/remove these large scale changes in brightness but I'm not sure how. I was thinking I could subtract a best fit plane from the original pixels - the best fit would be carried out as a function of dip angle or distance from the camera. However I have no idea of how to go about this, or if it is even a sensible solution.

An example of the kind of image I'll be working with is attached above. The original is much better quality but I had to compress it to upload it here.

Any suggestions or help would be greatly appreciated.

Thanks! Emma