I don't think he or his team did Hydrocity anymore, not after the fact that Miyoko Takaoka did Marble Garden. Neither
Hydrocity or
Marble Garden's songs were replaced in the PC version from 1996, when
every other confirmed MJ song
was. Not to mention the fact that both
Hydrocity and
Marble Garden have a legit Act 2 remix, when all confirmed MJ songs (and ones that are 99% certain to be his, but not confirmed) have Act 2s that are pretty much the same as the Act 1 arrangement, with just slight changes. (Carnival Night
Act 1 /
Act 2, Ice Cap
Act 1 /
Act 2, etc)
So who composed Hydrocity? Not confirmed in the slightest, but for some reason I'm thinking it was Masanori Hikichi, who worked at CUBE (
credited in the game) alongside Takaoka at the time (see Terranigma). I remember asking Masayuki Nagao (who was
credited alongside Hikichi in Mean Bean Machine in 1993, around the same time that work on Sonic 3 would have begun) if Hikichi and Takaoka worked on Sonic 3, and he said "I think so", which led me to contacting Takaoka, who confirmed her involvement. I haven't ever had the chance to ask Takaoka if that's his track, but she might not even remember and I'd rather not continue to bother her. If somebody can get a hold of Hikichi though, we could ask.
EDIT: Small detail, but Takaoka's Japanese Wikipedia page listing Sonic 3 is what led me to asking Nagao in the first place, if anybody cares. Seems like Takaoka put that info there herself.