question: what is your plan to finish the under hull?  Varnish or paint or carbon?  If you are not varnishing, you could mix up some epoxy and micro-ballons, fill and sand.  If you are vanishing, you could use wood flour (harder to sand) and you will see the patch.  Assuming it is symetrical (left to right) it will not effect the handling of your boat.  Personally I love those little imperfections, reminds me that I built it and it is not perfect ;)  I have a "dent" like that on the shear panels of my WD12 near the bow.  It was varnished so I chose to live with it, I've grown to like them and yes, they are symetrical :-D

Personally, I would not sweat it, I am of course assuming you plan to use this work and not display it in a museum.

You might be able to heat the area with a heat gun or a heat lamp (longer time) untill the epoxy softens a bit and then use some sort of custom cut brace from the inside to push the dip out and hold it while it cools.  Be careful to wrap the brace so it doesn't try to become part of the boat.  I have used this on smaller pieces.  Might work on a bigger one. You will then have a bit of leveling, sanding, filling to do but it might work.

I'm sure others will have other suggestions.

Ed

Hopefully, you've found a solution already. But if not you may want to try opening the seam at the center of the dip by drilling 1/16 inch holes along the seam close enough together to allow a hacksaw blade or fine tooth jigsaw blade to fit. Then carefully follow the seam, a few inches each way at a time, and watch to see if the bottom panels try to spring back up to a fair line. Be careful to make sure you know where the bulkheads are so you dont cut into them. If the bottom panels don't spring back into a fair line, you can try to wedge the seam open a little to help things along. Then just fill the seam with thickened epoxy.

 George W