If a liquid-gaseous interface were to exist on any of the giant planets, it should of course be located at points described by the usual state diagram. Which means, not all that deep into the planet. It would therefore not matter what kind of conditions were to be found at the core.

To elaborate, if the critical point of the ambient fluid (hydrogen, helium, oxygen or whatever) were to lie at 20 bars, then no surface of any "sea" could be found beneath the 20 bar level.