I'm searching for Sentinel-3 water data. But I couldn't find it. I've checked Copernicus Open Access Hub but it seems to provide only land data.
Yes, it's true. Water and land was split. The Copnicus hub provided land data and water data was provided by Eumetsat CODA. Now you can find both in the Copernicus Data Space Ecosystem
The water data is still available at the EUMETSAT Dta Store