The truth is that I have read a lot and I have become confused, the underground sea that Titan has, is it water or hydrocarbons, is anyone certain?
Underground liquid methane has been proposed as a source of the liquid in the methane-rich lakes on Titan, but it is not an ocean. Rather, as is summarized here it penetrates pores in the icy solid, similar to water in the pores of Earth's rock below the surface. The hydrocarbon liquid is not proposed to interact with the water (or water-ammonia) ocean below.
Cited reference: S. P. Falk, J. M. Lora, "Titan’s climate patterns and surface methane distribution due to the coupling of land hydrology and atmosphere", Nature Astrinomy (2019). https://www.nature.com/articles/s41550-019-0963-0
There seem to be two different kinds of liquid system operating simultaneously on Titan: one of these is the system of hydrocarbon lakes you are thinking of, the other though is much larger and underneath the visible surface crust those lakes rest on which, itself, is actually water ice that is so hard due to the extremely low temperatures (94 K) that it is about as hard and firm as rock - "water rock", so to speak, or "rockwater", if you prefer. But if you go deep enough down, it gets warmer, for the same reason as it gets warmer when going down into the Earth, and that means eventually you get up to the freezing point - 273 K(*) - of water at which point there is believed to start a more conventional water ocean, like those on other moons such as Enceladus, and Jupiter's Europa. The two systems are very different and physically incompatible with each other: the rockwater layer is what provides the necessary separation.
(*) Note: due to the overburden pressure, it will be somewhat higher. Also, it is believed this water will likely be a water/ammonia mix, so you should take this as more illustrative, the actual point at which the rockwater melts to water/ammonia mix will be lower, e.g. a 25% by mass solution of ammonia in water melts at about 216 K.