I have just had the same dilemma re waterline tiles and I realised two things (if it helps)... Firstly it's the colour of the coping that I needed to match to, or compliment, the tile. And secondly, the colour of the tile matters if your pool is fully tiled, but if you are going with pebblecrete, then it's the colour of the pebblecrete that affects the colour of the pool and not the tile! First time pool builder, so who knew?! My fingers did do a lot of googling to decide on pebblecrete colours though. Good luck!

Barnes.... I actually know of a product that cleans that off with ease!!! It's new has had testing and has been through the process of intellectual property and is now currently under production. If you are interested I can try source you some.

Potentially yes if they're cut/laid badly - I was involved on a job that $30K(ouch!!) of glass mosaic's were jackhammered out and completely replaced because the overall finish was deemed that poor and dangerous - the entry steps were too risky to use and there were tiles falling off before it was filled!.......That was a particularly bad/unfortunate case, and not wanting to completely scare you off the idea, if the the pool design has limited/deleted any underwater sharp corners or edges, and the glass tile install/prep is performed properly, then there's arguably low injury risk, and they can be a great finish if done correctly :)

I quite like the idea of using same tiles as you have around edge of pool. Here are some pics to help visualise how it might look. The last one has almost charcoal coloured tiles, which would look pretty good as contrast to your lighter tiles and the blue waterline mosaics.