From one of our plaster experts ....
Portland Cement does cure and hydrate for many years. However, it is best to allow curing to take place with plenty of moisture available so that it doesn't dry out.
Cement (and pool plaster contains a lot of cement in the mix) also shrinks when drying takes place. Therefore, it is best to cure underwater, but only after a sufficient amount of time to allow the immediate surface to harden properly. To do that generally requires at least 6 hours to a maximum of 24 hours depending on temperature and humidity. The higher the humidity, the cooler it is (as long as it isn't freezing), the longer you can wait.
Many plasterers are afraid of the plaster cracking due to drying out, and therefore, start the water immediately after finishing. But that is not the best practice. Waiting six hours is not going to make a significant difference. If the plaster mix is thick (with a low water to cement ratio), and with no water troweling, and no calcium chloride added, then the chances of cracking are very minimal. Placing a tarp over the pool is also a good practice in hot and dry weather to prevent cracks.
