While it is theoretically possible for a planet to be composed mostly of water, a planet entirely made of water with no solid core or landmass is highly unlikely to exist in reality. Planets in our universe typically form through processes such as accretion, where solid materials clump together to form a core, and then attract surrounding matter to build up a solid or rocky mantle and crust.
Water-rich planets, often referred to as "ocean planets" or "water worlds," do exist in scientific speculation and are subjects of study and exploration. These planets are believed to have a significant amount of water on their surfaces, potentially even with a global ocean covering the entire planet. However, it is thought that beneath the water, there would still be a solid core composed of rock or metal.
The formation of a planet made solely of water without any solid core is currently not supported by our understanding of planetary formation. The dynamics of such a planet would be challenging to explain within the framework of known astrophysical processes.