Part of it will be a function of surface area of cement vs volume of water. If you have a ton of water, a little bit of cement isn't going to affect it much (especially once it's cured), but if you have the opposite, and have a ton of cement and only a little water, you might see problems. Keep in mind that cement takes a very long time to cure. Technically, it takes several hundred years, but several months to a year would probably be enough time for it to be cured enough. Cement will continue to cure even under water.