Ponding on shower windowsill

[Bienes Raices]

There is a small window in the shower and water is ponding on the sill when the shower is used, which apparently is causing the shower wall tiles to deflect below (most of them now need to be replaced). I was told that the sill needs to be re-installed in a different way to prevent this from happening in the future, but I'm not sure how to do that. It has caulking around the sill now but the water is still getting in. Thanks.