Aquazone? 1995-1998 I think. I bowed out into a dark age in 1997, so I missed the later Aqua Raiders and Hydronauts stuff.
First 2 years were kino:
1995 and 96 -Aquanauts vs Aquasharks - clear good guys and bad guys, best submarine designs
1997 -Aquaraiders was a third faction who were villains, but didn't care and fought the Aquasharks as well - only the small sub with the drill arm was good and the theme had only three sets. I forget if it was this year or the next, but they came out with ANOTHER villain faction called the Stingrays who were these weird mutants with iron helmets that weren't quite classic hardhat diving helmets, green and red torsos and brown pants. The color scheme of their sets were about the same as UFO.
The Town "Divers" theme came out in '97 and was a realistic scuba theme. Pretty comfy, but lacked the adventurous feel Aquazone had. They introduced a ton of new sea creatures though like dolphins, stingrays, white sharks, and saw-nosed sharks.
Like I said, my dark age started after that, so I can't really tell you about anything that came after that. I agree with the thread subject though. When I look back at old Lego, that period is the most appealing to me.