Help!!! Filling holes in the hull...

I need to replace my swim step. I had a solid wood wrapped in fiberglass that fell off last time I was at the river.

I'm now thinking of using 2 smaller steps instead of 1 long one. I'm going to need new brackets as well and here lies the problem.

The 2 brackets on the left are spaced 14 5/8 inches apart and the ones on the left are 15 1/2 inches apart. Basically they are off and I'm going to need to screw new brackets in that match the new swim steps.

What should I use to fill in these holes?
They are below the waterline and do not go all the way through the hull.

http://images.tapatalk-cdn.com/15/06...1ea16d887d.jpg

http://images.tapatalk-cdn.com/15/06...9da51ecd01.jpg

http://images.tapatalk-cdn.com/15/06...8141d416b4.jpg


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