As the comments above indicate, factors like density, pressure and temperature are important for a Jupiter submariner. Of course nobody yet has the exact details of Jupiter's interior structure, but there's a diagram in [this LASP page](http://lasp.colorado.edu/education/outerplanets/giantplanets_interiors.php)<sup>[[WebCite archived version](http://www.webcitation.org/6J1dhwh3H)]</sup> that indicates the following:                                                                       At the intersection of Jupiter's liquid hydrogen/metallic hydrogen layer, the density is about $1\text{ g}/\text{cm}^3$, which would permit the submarine to float. However, the temperature there is $\sim5000\text{ K}$, and the pressure is $\sim2 \times 10^6\text{ bar}$, so it can't be done with normal equipment.