1) Tape or sticker

You can sticker or tape the area as a temporary fix. If there is no actual breach in the hull, this might be the best option here, as you can always take your repair further if needed.

https://www.amazon.com/Surfboard-Repair-Kit-Ropers-Patch-10/dp/B00KLNAB3W

https://www.amazon.com/Clear-Ding-Tape-Single-48mmx4m/dp/B012XL4NLK

2) Paste Epoxy or sealant

Marine centers will have past compound kits that you can rub into dings, these work in water as well. When doing a long voyage I carry some with me.

https://www.amazon.com/3M-Marine-Adhesive-Sealant/dp/B07M87ZPM8

https://www.amazon.com/WEST-SYSTEM-105-K-Fiberglass-Repair/dp/B007D2QV5Q

3) Epoxy Resin

This will take more time than the other options above, but will be closer to a complete repair. I suggest SolarRez, a UV activated epoxy resin. You can purchase on Amazon, marine center or surf shop should all have or similar.

https://www.amazon.com/Solarez-Cure-Epoxy-Ding-Repair/dp/B0007UC4MU/ref=sr_1_1?keywords=solarez&qid=1559425334&s=office-products&sr=8-1