The female swan had a netted bag, used to hold footballs, wrapped around her head and wings at Wyken Slough nature reserve in Coventry.
She remains at Wychbold Swan Rescue, near Droitwich, Worcestershire, following Saturday's rescue.
The RSPCA said fishing line and hooks were also "dangerous hazards".
The swan, which had been seen struggling to keep her head above water, sustained damage to one of her wings and remained under observation on Friday.
RSPCA inspector Helen Smith said the swan was "in complete shock" when she was rescued by members of the West Midlands Fire Service.
"She could have been struggling to free herself for some time and was clearly exhausted," she said.
"The netting... was so tightly wrapped around her body she was in real danger of drowning."
The RSPCA inspector reminded members of the public about dangers to wildlife and wild birds.
She said: "Netting, litter and fishing line and hooks are dangerous hazards to wildlife and I urge people to think of the consequences before they casually discard such items."
A warning has been issued about the dangers of discarded netting and litter after the rescue of a swan which was in danger of drowning.