I'm assuming you mean white painted fiberglass steps, they might of cracked from age or improper support. Anyways to fix it permanently would be to use fiberglass and then pool paint to match the white color after your done. This would require draining though. Any other material is kinda temporary since the steps flex when you walk on them, AB putty cracks off, anderson white flexible sealer might work too, Plast-Aid might be a good choice but also becomes very rigid and could crack off. If you are going to replace the steps entirely just make sure to properly support them, When I need to make a permanent repair I prep by sanding the crack and even using dremel then use about 5 layers of fiberglass (force out any bubbles) then I sand smooth and paint with pool paint. The end result looks pretty nice although you loose the diamond pattern in the repaired spot.