Here is one attempt using layers in GIMP 2.9:
Basic idea is to first remove the color from the lens flare, adjust the luminosity and then put the required color back in.
Step 1. Duplicate layer twice and desaturate top layer (I used Luma mode)
Step 2. Add black layer mask to BW layer and using soft brush paint paint white on the lens reflection area. Merge down. Now you have original layer and the one with black and white area where lens reflection was.
Step 3. Create a new transparency layer and using the dropper fill it with the brown color of the jetty. Add black layer mask. change the layer mode to color (HSL).
Step 4. Paint white (with low apcity brush) on the black layer mask and adjust opacity to taste.
I did the whole thing a bit fast. With more care, I am sure you will get far better result.