If your pond is truly made of fiberglass, then you might want to learn how to use fiberglass sheets along with the proper resin. That would be the best fix in my opinion. I learned how to do it back in high school auto-body shop in the '70's.
if you're handy, it's not hard.
If you put caulking or putty in there, it will probably just crack again.
You can buy kits at auto parts stores, marine stores or online.
I'll bet you can find a bunch of You Tube videos on the subject.
Basically you clean and rough up the crack and surrounding area. A grinder works good for this. Use eye protection, gloves and a respirator or a dust mask. With a scissor, cut a fiberglass cloth to cover the crack with a good amount of overlap. The resin is two parts, resin and hardener. Mix the proper amount of hardener with some resin in a disposable container. A tin foil pan is good. Spread some of the mixture onto the area with a plastic squeegee. Lay the cloth onto it and squeegee it in place getting all the air out. Squeegee some more resin on top of it until the cloth is saturated and your done. You have to work fast because it sets up in minutes. You have to wear disposable gloves.
If the edges turn out rough when it sets up, it can be sanded. You can even add additional layers later if you want.
This will be a solid permanent repair if done correctly.
It can be painted, but in a pond I would just leave it bare. Algae will grow and cover it quick enough.
