The UN's Maxwell Gaylard made his appeal by one of Gaza's sewage lakes
The UN and international aid agencies say Israel must relax its blockade of the Gaza Strip to allow urgent repairs to the water and sewage systems.
In a joint appeal, the bodies say the hazards to health and the environment threaten not only Gaza but Israel too.
More than 13m gallons (50m litres) of raw or partially treated sewage flows into the sea every day from Gaza because of a lack of treatment plants.
The cross-border aquifer is low and raw sewage floats back to Gaza and Israel.
Deadly flash flood
The UN says about 10,000 Gazans have no access to a water network - while about 60% of the 1.4m population receive water only intermittently.
Water consumption in the Strip is less than a third of that of Israelis living just a few kilometres away.
Israel, and Egypt on its south-western side, have kept Gaza largely sealed since a violent takeover of the territory by the Islamic militant Hamas group in 2007.
Israel says it is trying to weaken Hamas, end its rocket attacks against Israeli towns and get back an Israeli soldier, Gilad Shalit, who was captured and taken to Gaza in 2006.
"The deterioration and breakdown of water and sanitation facilities in Gaza is compounding an already severe and protracted denial of human dignity in the Gaza Strip," said UN humanitarian co-ordinator in the Palestinian territories, Maxwell Gaylard.
Mr Gaylard and other humanitarian workers and officials launched the appeal with a news conference near one of northern Gaza's sewage lagoons to highlight the problem.
In 2007, one of the lagoons overflowed and five people were killed by a flash flood of sewage.
Aid agencies said Israel's bombardment in December and January worsened an already bad situation.
Israeli officials had no immediate comment to the appeal on Thursday.