There are at least 9 groups with the name H2O (not counting H₂O):
1. A melodic hardcore band from the United States
2. A pop rock band from Latvia
3. A hip hop duo from South Africa
4. An alias of Oliver Strumm
5. A rock band from Scotland
6. A hip-hop/dance group from Montenegro
7. A techno act from Norway
8. A progressive rock band from Italy
9. H2O was a Japanese folk/pop duo from Nagano Prefecture: Kenji Nakazawa(中沢賢司） and Masaki Akashio(赤塩正樹).
1) H2O is an American melodic hardcore/punk band from New York City that was formed in 1995 by Toby Morse, a former roadie for legendary hardcore act Sick of it All. They initially released their self-titled debut on Blackout Records before signing to punk powerhouse Epitaph Records for their breakout Thicker than Water. The band recently released their Bridge Nine debut Nothing to Prove, which was met by considerable fan and critic acclaim as a comeback of sorts. H2O also somewhat recently released a DVD titled One Life One Chance before signing to Bridge Nine. Their music has been featured in the video games Street Sk8er and Crazy Taxi. ~ Wikipedia
2) H2O is a Latvian pop-rock band. In 2006 they participated in Eirodziesma 2006 (Eurovision Song Contest national selection in Latvia) with the song "You Say That Kissin’ Is Good". In 2007 they released their debut album "Uzmini nu". The members are Jānis Strapcāns - vocal, Staņislavs Judins - bass, Mārtiņš Miļevskis - drums. More information here (in Latvian).
The band has recently begun cooperation with the music publisher and management company MicRec PM. Their current single "Uzmini nu!" ("Guess what!") has permanently settled among the top five songs in Radio SWH's Top 40 (two weeks at #2) for seven weeks. The song was written by Staņislavs Judins (bass) and Jānis Strapcāns (vocal). The producers are Gints Stankēvičs and Mārtiņš Freimanis. H2O's song "Jaunatnes dēļ" ("For youth") has been chosen as the title song for the recently presented film "Jaunatnes likums - ikvienam jaunietim!" The band has existed for three years and has regularly participated in the Bilde’ music festival. They have appeared as the guest artists on the reality show Talantu Fabrika as well as on other TV programmes, and have been a warm-up band for Credo. H2O's most popular songs "Kā klusums kliedz" ("How the Silence Screams") and "Mīnus minūte" ("Minus Minute") have also been successful on Radio SWH. With "You Say That Kissin' Is Good", written by M. Freimanis, the band came fifth in the 2006 Eurovision National Final. The band's founder and lead singer Jānis Strapcāns gained useful musical experience in the USA, where he participated in a three-month music seminar and attended vocal and guitar playing lessons conducted by professional musicians (including Grammy Award winners). Recently he was one of the 12 finalists who competed for the post of MTV Latvia veejay.
3) A South-African Hip-Hop duo (Menzi Dludla and Sphiwe Norton) from Meadowlands, Soweto. They initially started off making kwaito music but then moved over into Hip-Hop releasing tracks on various compilations as well as their own albums on the Outrageous records label.
4) An artist credited on the album "AnotherLateNight", also known as Oliver Strumm. He has a discography page here
5) H2O were a Scottish band who formed in Glasgow in 1978. They were led by lead singer Ian Donaldson. Their biggest success was the synthpop single "I Dream to Sleep" which reached number 17 on the UK top 40 in May 1983. The follow-up single "Just outside of Heaven" reached number 38 later the same year. The following year they released their one and only album Faith, but further success eluded them and they split soon after. The band also featured Ross Alcock on Keyboards, who participated in a large amount of the band's songwriting.
Donaldson had a minor solo career in Scotland and is currently in Scottish group Fourgoodmen alongside Derek Forbes, Michael MacNeil and Bruce Watson.
6) A Hip-Hop/eurodance group from Bar, Montenegro. Their most notable hit was 'Ti Nemas Prava Na Mene'.
7) A norwegian techno act. H2O is Stephan Groth and Geir Bratland, both members of Apoptygma Berzerk. They released two singles: "Wasteland Beats 1 Dre:amseller" (SpaceEcho Rec. 1994) and "Fahrenheit" (SpaceEcho Rec. 1995).
8) An italian progressive rock group. The band was formed by Claudio Andreotti (bass), Rocco Malaguzzi (guitars) and Luca Prandi (keyboards and vocals). They released 2 albums before disbanding: UnoPuntoSei (1997) and Due (2001).
9) H2O was a a Japanese folk/pop duo from Nagano Prefecture: Kenji Nakazawa(中沢賢司） and Masaki Akashio(赤塩正樹).They began the band back in 1976 before breaking up in 1985, but near the end of their 9-year run, they released the song for which they'll always be remembered for: " Omoide ga Ippai" (Full of Memories).