Four men had apparently gone into a lake at Brookvale Park, Erdington, Birmingham, to rescue a football but only three could get out of the water.
Three men are thought to have gone into water at The Station Fields, Pelsall, and only two made it out.
The ambulance service said searches were due to resume on Monday morning.
"We must use these incidents to highlight the dangers of going into water on warm days"
A spokesman said it was called to Erdington, where the water was waist deep, at 1835 BST on Sunday and the search was called off for the night at 2120 BST.
He said: "Fire crews arrived to mount a search of the lake and the police helicopter used its thermal imaging camera but to no avail.
"The three other men who escaped from the lake were cold and wet and were treated at the scene by ambulance crews.
"They were extremely shaken and upset by what had happened but did not require hospital treatment."
Men in their late teens or early 20s are believed to have been involved in both incidents.
'Strong currents'
Fire crews and a police helicopter were also sent to the area off Ryders Hayes Lane in Pelsall to try to search for the second missing man.
The search there was also scaled down at sundown.
Trevor Ford, from West Midlands Ambulance Service, said: "We must use these incidents to highlight the dangers of going into water on warm days.
"Especially at this time of year, the water is very often not as warm as you might imagine and this can lead to people getting into difficulties.
"Equally, those who cannot swim should be extremely careful in such circumstances. Some rivers in this area do have strong currents.
"Diving into water presents its own dangers with underwater objects a real concern."
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