What does your contract say? I know mine says that any work subcontracted by me and not the pool builder is not warrantied, it's not clear though what the knock-on effect would be. I.e. if that voids any warranty of the builder - I don't read mine that way and would see it as only that subcontracted work not being under warranty.

The issue I see for you is in the event you do have damage to the shell say, the pool company trying to say it was caused by the tile contractors being in the pool.

Do you have anything in writing about the breakdown cost of installation for the current tile amount? If you do, you could use that argue the cost of the install.

Sorry you're having issues!

Most contracts will state that they will not warranty the work that was done by others or the damage done to the pool because of another contractor's negligence.

All in all that's not a bad price for tile installed in a pool. The caveat to that is if he is doing it properly. Is there multilayer water proofing? You have to remember that tile or tile grout is not waterproof. Even high strength shotcrete in permeable to water. Waterproofing the shotecrete, brown coat and grout all help to prevent leaks. It does cost money to buy water proofing materials and the labor to apply them. If the work is above the square footage of what you agreed to then the cost of installing it is not out of the ball park if its done properly.

Your tile guy that you hired for your kitchen may not know how to install it inside a swimming pool. A lot of pool builders do not know either. Your going to find out if you have a good one or not by the way he installed the first tile.

Hope this helps.