Comets add some water either through direct hits or fragments entering the atmosphere.
The amount after the great bombardment in the 3.8-4.1 billon year ago epoch is negligible relative to the hydrosphere at that point and today the mass has not largely changed. Even water in the sediments in a descending oceanic trench plate is returned to the hydrosphere via volcanoes along the ring of fire and volcanic hot spots return mantle waters too.
NASA has been studying comet isotopes they differ.From earth’s ocean based on where the comet is from in the solar system.
https://www.jpl.nasa.gov/news/comet-provides-new-clues-to-origins-of-earths-oceans/