What is the Best mixture ratio of Portland cement Quartz sand and bonding agent for under water application
Are you sure you don't want to be looking at a hydraulic cement or mortar rather than just Portland cement?

http://www.lowes.com/pd_41334-96-00924_0__?productId=3033247

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&amp;rt=rud