Consider tile porousity and water absorption. Water absorption can cause freeze/thaw spalling issues, etc.

When installing, use a full coverage of thinset behind the tile. No voids.

Surface texture. Glazed tile would be considered more sanitary than a textured stone, there's less of a chance of floating debris collecting, drying, and creating a "scum line" at the water line.

With all that written, I'm in New England, I have a tight-grained slate as my waterline tile. No issues over 15 years.

Maintain your water quality and you'll be surprised how few issues you'll have with your pool. Clean and properly balanced water makes pool life easy.

Good luck.

Where do you live? There are different issues based on climate.