Motifs in music can sound similar, and sometimes be mixed up for melody. A theme that is aquatic based is going to have a certain sound to it. (It would not be surprising to me that SG: Atlantis and SeaQuest DSV would have a similar sounding motif). A lot of times, it is the same for space based sci fi action fare.
Take Star Wars, Battle Beyond the Stars, Battlestar Galactica (the original series), even a couple of Star Trek themes....you'll hear similarities in motif.