My pool has steadily been shedding waterline tiles. Mostly the narrow rectangle ones at the top. I have a small bucket full of them. Only about 8 of the main waterline tiles have come off in once section. A bunch look they were just siliconed in place by the prior owner. I want to put them back on correctly with motar. What do I use? Do I need to drop the pool level way down? The upper tiles are above the waterline.

Here's a pic from last year. The top row of tiles on the far side- you can see how the grout changes color in sections where they "glued" tiles back in place.