The most commonly tiled pools are referred to as competitive pools; like what you'd find in a high school or college natatorium. These are almost always
1 x 1 mosaics, porcelain or highly vitrified. As an aside, this size is used because of the critical dimensions required for these types of pools.
However, I'm going to disagree with Davy on both counts. I would not use modified thin-set or epoxy grout in continuous immersion. Robert (Nforcer2) points out that Laticrete 254 is suitable for underwater applications. I'd guess that this is one of the few modified thin-sets that are acceptable in immersion applications.
Otherwise, a high quality unmodified thin-set will do fine. If you feel that you need latex, use the same manufacturers' liquid latex admix if they'll approve it for underwater.
I can't speak to using marcite, I'll defer to poolman's expertise there. But most pools I've ever seen were grouted with the good old job mixed sand and cement at 1:1 ratio.