I'm installing a ceramic mosaic in the bottom of my concrete pool and wondering what is the best mixture of white Portland cement Quartz sand and a bonding agent to apply this mosaic. Can I just use white pool plaster to install?
Are you sure you don't want to be looking at a hydraulic cement or mortar rather than just Portland cement?
Or something like this? http://www.superior-industries.com/pro_patch_600_product_271.html
I'm not trying to advocate specific products or brands, but illustrating examples of products that are actually designed for underwater use.
If you were looking at one-off repair of loose or detached tiles, you might consider an underwater adhesive: http://solutions.3m.com/wps/portal/3M/en_US/Adhesives/Tapes/Products/~/3M-Marine-Adhesive-Sealant-5200?N=5510818+3294314622&rt=rud