There are several bands named Poseidon:
1. A german prog/krautrock band formed in 1974 who released a single album in '75 called "Found My Way". The album got re-released by german label "Garden of Delights" with seven bonus tracks in 2001. The band was:
Horst Meinzer (vocals, bass, guitar), Tony Mahl (keyboards), Wilfried Sahm (guitar), Theo Metzler (guitar) and Rudi Metzler (drums). 2. A three piece hardcore band from Bremerton, WA.