The final Aquazone subtheme in 1998 was the Stingrays, which were fish-like mutants and enemies of the Hydronauts who used underwater vessels that resembled sea creatures. The villains included Commander Manta Ray and Raven Ray and the subtheme contained 5 sets and set 6109 Sea Creeper was the same as set 6140.
Aquazone was an underwater theme introduced by LEGO in 1995 that lasted until 1999 and contained five subthemes all of which took place in the ocean and typically involved two sides fighting each other for valuables such as Hydrolator Crystals.