Have you seen supercritical water and/or helium? The “surface” of Jupiter is probably supercritical hydrogen. I don’t know if there’s a sharp cutoff like Earth’s oceans or a gradual thickening, but it’s still only half the density of water. It’s possible to build a boat for that!

However, the pressure would be around half a million bar, or 500 times the pressure of the deepest part of the ocean. This is also 5× the pressure used to make synthetic diamods too, and probably about the same temperature too. If the boat had any grease left outside, it would be diamond grease at this point.

If you went further down to where the density increases to about the same as water at sea level, the pressure would quadruple to nearly the same as Earth’s core, and the temperature would be about the same too. At this pressure, there’s probably another indistinct boundary of metallic hydrogen, and if the boat has survived the ultra-high-pressure hydrogen embrittlement, the steel-liquifying temperatures, and diamond rain, this metallic hydrogen will almost certainly reduce it to a lump of novel metal hydrides.