Gunite is not water tight. I would be concerned the water would seep into the surrounding ground and make it a mushy mess. They want the gunite damp for plastering, not saturated. It may affect the integrity of your plaster finish if the gunite is saturated.
Valid reasons.
Specific to the curing of the gunite it is not more or less beneficial having it under water when its 100+ outside?
What is the thing down by the water? Is it a pump? It needs to go for the many reason stated above.