Water was needed for cooling the electronics of the Lunar Module. To remove excess heat, water was evaporated by sublimator plates into space. Without cooling water, the electronics mounted on cold plates may overheat and damaged.
SUBLIMATORS The porous-plate-type sublimators (one in the primary loop
and another in the secondary loop) are identical, except that the
primary sublimator has a larger capacity. Each sublimator has a
coolant inlet and outlet, a water inlet, and a steam outlet. Water
makes one pass through the unit; coolant makes six passes through the
primary sublimator and four passes through the secondary sublimator.
For proper sublimator operation, water pressure must exceed 4.0 psia,
but be less than 6.5 psia. The water pressure must also be less than
the suit circuit static pressure plus the head pressure from the water
separators to the sublimator.
The unit rejects heat to space by sublimation of ice. Water from the
WMS flows through the water passages, into the porous plates, and is
exposed to space environment. The vacuum pressure is below the triple
point of water; this causes an ice layer to develop within the pores
and on the inner surface of the plates. As the hot coolant flows
through the sublimator passages, heat transfers from the coolant to
the water and to the ice layer. The ice sublimates from the solid
state to steam without passing through a liquid state, rejecting its
heat load overboard through a duct. The thickness of the ice layer
varies with the heat load imposed on the sublimator, resulting in a
regulated output temperature over a range of input temperatures.
COLD PLATES AND RAILS
Electronic equipment that requires active temperature
control is cooled by cold plates and cold rails. Most flat cold plates
are installed between electronic equipment and the LM structure in a
manner that minimizes heat transfer from the structure to the coolant,
to avoid a reduction of the coolant cooling capacity. The surrounding
structure and equipment may have a temperature range of 0 F to +160 F.
The remaining flat cold plates are installed directly on the
electronic equipment without making contact with the LM structure.
Cold rails are also structural members and are used in the aft
equipment bay in the descent stage for the DSEA. The IMU has an
integral cooling circuit. Cold plates and cold rails for equipment
essential for mission abort have two independent coolant passages, one
for the primary loop and one for the secondary loop.
Graphics and block quote from LM intro PDF and
ECS description.
ECS = Environmental Control Subsystem
