Heya,
Yes, you need to research Landfill2, which will unlock deep-water-landfill, which requires landfill, concrete and sand to make.
Landfill will allow you to fill in coastlines, which is handy early game where you need to squeeze in things, and later game you get access to being able to fill in big swathes of water, but you need both landfill and deep-landfill to fill in a single tile. So you need to be more thoughtful about how you use it.
