I agree with japb. Most pools in Fla. have a cantilever deck that sits on the beam of the pool. The joint is commonly called a stegmeier joint but other terms have been used. This joint is usually filled with a product specifically designed for this type of joint. Do not use the self leveling products here, they will run down the tile and make a mess. This should not be a grouted joint. Good Luck!

Either tile choice will look nice. The white plaster will cast a light blue color. This depends on your pool location and when it's in full sun and when it's in the shade. For a more pronounced blue, some finishes from Diamond Brite such as Blue or Cool Blue, or a premium polished finished from Hydrazzo such as Hatteras Gray, which actually casts a blue color, will look very nice. White grout for the tile.

What did you decide? I am grouting my pool waterline and trying to decide what to do. I have a light glass waterline tile, that looks almost white/off white and I'm trying to decide if I go with a dove gray or lighter gray like silver. Send photos!

This can't be evaluated properly at a distance because there's no guaranty that anyone's monitor is going to reproduce true colors. You can only get estimates. I would go with grout that most closely matches the tile that's being grouted. With different tiles colors you have to pick your poison.