A Reiquarium is a self-contained body of water containing Rei Ayanami
and/or her clone(s), a character from the anime Neon Genesis Evangelion
-- be it a hot tub, tank (see episode 23), swimming pool, pond, lake, sea, ocean, or water world.
Lengths of running water such as brooks, creeks, streams, and rivers containing Rei Ayanami
and/or her clone(s) ARE NOT Reiquariums because they are fed from and empty into self-contained bodies of water (see above).
Misato took the three Children and Pen Pen on a vacation trip to Okinawa. While Misato, Shinji, and Pen Pen had fun on the beach, Rei and Asuka went SCUBA diving -- thus turning the East China Sea into a Reiquarium.