I checked their site and I think underwater just restyles the HS typesetting and font right? I wasn't sure if there was any additional editing or better raws than HS uses. Im not too familiar with the group
Underwater always uses the highest quality video source available, be it TV broadcast, DVD or BD. In this case, it's the TV Tokyo broadcast. So in other words, it's not CR's video and looks better than CR's video. Other than that, the treatment subtitles go through generally means re-styling, actually typesetting the signs and QCing. Opening and Ending lyrics+romaji are also added whenever possible. Besides that, there's also chapters, something that all direct CR rips (HorribleSubs, CrunchySubs, FUNiGuys) lack.
(And for the record, I am the leader and only active member of Underwater - it's pretty much a one-man effort 99% of the time, with some additional helpers from time to time)