Yes, our tiles are vitrious, (glass-like). Water cannot penetrate to cause problems in areas where frost or freezing is a concern. This makes them well suited for water areas such as pools and fountains.
Usually 10% is recommended. If your project has tight areas or a detailed layout you may want to order more. Always good to have a stash saved in case of damage from leaky pipes or unexpected remodel by 6 year old on a skateboard...
We recommend natural gray grout, it's so boring your eye doesn't notice the grout, just the tile. Although if you wish, you can darken it a bit with a natural gray/charcoal blend, maybe 3 parts to 1 or warm it a bit with a natural gray/brown mix.