The problem with Einsteinâ€™s gravity model is the the time/space is one dimensional, like a stretched bed sheet which is indented by the mass of an object placed on it. Space is three dimensional, so the time/space fabric is more like being under water. How would mass make a dent under water?

Yeah its quite complicated while understanding spacetime. Because first of all non euclidean geometry is used in this theory and it is a 4D spacetime so it is a bit difficult to imagine and could only be understood by mathematics. Over to your last sentence you said see its water and space time is bit different but you can imagine while a planet is in spacetime it creates a distortion in 3d space… For this you can see gravitationl lensing around black hole or else watch this simulation just for approximate understanding https://m.youtube.com/watch?v=3gky7oOxCAM

The problem with Einsteinâ€™s gravity model is the the time/space is one dimensional, like a stretched bed sheet which is indented by the mass of an object placed on it. Space is three dimensional, so the time/space fabric is more like being under water. How would mass make a dent under water?

Yeah its quite complicated while understanding spacetime. Because first of all non euclidean geometry is used in this theory and it is a 4D spacetime so it is a bit difficult to imagine and could only be understood by mathematics. Over to your last sentence you said see its water and space time is bit different but you can imagine while a planet is in spacetime it creates a distortion in 3d space… For this you can see gravitationl lensing around black hole or else watch this simulation just for approximate understanding https://m.youtube.com/watch?v=3gky7oOxCAM