There are a number of things you could use to repair a crack in the plastic steps in your pool. You could use pool putty for example.
Yes it will crack. I think any and all concrete will crack. Maybe not enough to leak water but hairline cracks. Coat your pool with good waterproof sealant if leaks start.
If the crack is small I would suggest buying a patch kit from a local pool store. The patch kits are easy to apply after the pool is drained. If the crack is large you'll need to hire a professional. You can call someone and they will fix your fiberglass pool. You can also try to buy a repair kit depending on how big the crack is. If it is a big crack, your should let a professional do the job.
How to repair vinyl liner pool stepsYou can use fibre-shelkote putty and gel coat. Usually a crack is due to loss of underlying support, just putting a patch over the crack will not suffice. Contact pool step specialists who repair just swimming pool stairs to get the proper tread repair completed, or to order a kit. Try Poolstepspecialists.com
you can either use a good epoxy pool paint if you intend refurbishing the whole pool or use an epoxy paste. V out the cracks and fill with epoxy paste.
jb cold weld or any epoxy for plastic/fiberglass material
If the crack(s) are small and you have determined that your pool is not loosing water then leave the cracks alone. If the crack is sufficient to make the pool loose water ( determine that you are actually loosing water with a bucket test) the crack can be sealed with epoxy putty, fix-leak, or hot epoxy. It takes a specialist to determind just how much of the crack needs to be reworked.
A crack in your swimming pool indicates a structural defect. Contact the pool manufacturer - your pool may still be under warranty, meaning you could get it repaired or replaced for free. If the pool is not under warranty, contact a pool repair specialist; you may want to get estimates from three or four people before you hire someone to make repairs.
If your crack is losing water, chewing gum works well (seriously), it will adhere under the water, and flexes with the crack when bathers put their weight on the step, then the chewing gum will reseal itself until you are able to repair the crack properly. Underwater epoxy can also be used, but gum is cheaper, and flexes whereby the underwater epoxy hardens and doesn't flex. Pool StepSpecialists.com has kits for fiberglass pool steps. Remember to chew the gum first, then push it into the crack(s)
There are 2 part epoxies that will do the trick. The product has a clay like consistency to it and the label should tell you if it can be used underwater. Pool stores, Lowes, Home Depot all have a suitable product. Clean the crack of debris or loose plaster, mix up the epoxy and push it into the crack as deep as you can. Go back over the area with a finger to remove most of the excess and give a decent appearance to the repair. It will last until whatever caused the crack allows it to crack again. There is always a reason a pool cracks....expansive soil, ground movement, trees (I am fixing a tree crack right now) soil compaction/liquifaction etc.
If the pump starts shutting down when you try to use other skimmer box it may have a hairline crack and will normally happen when you try to vaccum