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 http://lasp.colorado.edu/education/outerplanets/giantplanets_interiors.php that indicates the following:                                                                       At the intersection of Jupiter's liquid hydrogen/metallic hydrogen layer, the density is about $1$ g/cm^3, which would permit the submarine to float. However, the temperature there is $~5000$K, and the pressure is ~$2 \times 10^6$ bars, so it can't be done with normal equipment.