- The Hydronauts were called Aquanauts in catalogues outside the U.S., suggesting a continuation of the 1995 Aquazone faction of the same name (Aquanauts).
- The subtheme's colour scheme was similar the one of the earlier Aquanaut line, the only differences were the transparent pieces which were now exclusively transparent green instead of blue and trans-neon orange.
- The minifigs had unique masks printed onto their faces which gave them a very alien look.
- The Hydronauts used suction cups to move their storage boxes in contrast to the commonly utilised magnets.
- The 1999 set 1095 Super Sub featured a minifig with an Aquasharks torsopiece and didn't match the colourscheme of the 1998 Hydronauts sets. The box however, was labeled with the Hydronauts logo. The set is the same as 6100 Aquashark Dart from 1998 which was then attributed to the Aquasharks.
|Image||Item number||Set||Number of pieces||Minifigures||Price||Release|
|6180||Hydro Search Sub||274||4||$50.00||1998|
|6199||Hydro Crystallization Station||472||5||$89.99||1998|