Underground sea in Titan(Saturn's moon), water or hydrocarbons? Is the underground sea on Titan formed of water or hydrocarbons. Is anyone certain?

I have read a lot of various documents from NASA and Cassini missions and Huygens missions, and I have become confused.
Titan is believed to have a layer of liquid water under its surface, due to tidal heating of the icy crust.

Titan has a surface formed of ices with some hydrocarbon (methane) lakes. The crust of Titan is mostly water ice.  Below this, there is a liquid water layer and perhaps more ice and silicate rocks below that.

This is described at https://science.nasa.gov/science-news/science-at-nasa/2012/28jun_titanocean