In the (English translation) of the novel, Solaris is described as covered completely by an "ocean", but parts of the surface are constantly changing as the planet appears to experiment with creating objects from the memories of the humans on the space station, such as landforms, structures, and of course, the resurrected humans.  So it's not a water ocean, it's a sentient thing.  IMHO, the novel is better than either film, but thematically, Soderbergh is closer to the novel than Tarkovsky, who really had no interest or apparent understanding of what Lem's book was about.  That doesn't mean Tarkovsky's film isn't a great piece of cinema (I think it is); it just isn't really representative of the novel.