“The sea” is a term commonly applied to the Dead Sea. Compare Deuteronomy 3:17, “the sea of the plain (Arabah), even the salt sea.” The more literal rendering of the verse in this sense would be, “and go into the sea; into the sea go the waters that issue forth, and the waters shall be healed.”
Healed - Every living thing (of which there were none before) shall abound in the “healed” waters. The absence of living creatures in the Dead Sea has been remarked by ancient and modern writers. So the water which Jesus should give should bring life to the dead in trespasses and sins. Compare John 4:14; Revelation 22:2-3.