The quickest and most effective way is to have the grout cut out, that is the reason i mentioned to get an agreement about damaged tiles. If your pool has a foot print of 39sq M's, taking into account the walls, this will give you a total area of approx 80sq M's. Also remember when you get your quote, make sure you get them to include cleaning the pool after the grouting has been finished, it's a nightmare to get the grout dust off. From start to finish, the work should be completed (allowing for the grout to completely harden)in a week.