This is primarily done using skimmers which float along the surface of the water ... conveyor belts of sponges of absorbant sponges, and 'weir' skimmers which sit just below the waterline and ...

After you've filled all the joints, make your first “cleanup” passes with the float. Once the grout has started to harden (20 to 30 minutes), begin sponging. Don't use just any sponge ...