David Briffaut, 23, lost consciousness and suffered serious injuries after hitting the water in a pool at the bottom of a slide at the Aqualandia tourist attraction in Benidorm.
He is currently on a life support machine in a Spanish hospital and his family fear he will be left paraplegic after scans showed he has two broken vertebrae in his neck.
His parents, teacher Lorraine and construction worker Stephane, have flown out to bring Mr Briffaut, who works as a green keeper at a golf club in Essex, home.
Mr Briffaut's uncle, Mark, said: "This is every family's worse nightmare. David is a wonderful young man, who was enjoying an innocent day out with his girlfriend.
"We are praying for a miracle, but we have been told that the injuries he has sustained are very traumatic.
"We cannot understand how this happened at a family tourist destination. David was behaving in the normal way, and he had not been drinking.
"We believe there should be a full investigation into the circumstances of what happened."
Mr Briffaut was on holiday with his girlfriend of six years, Penny Bristow, at the time of the accident on the "Splash" slide.
They had been staying with friends and were on holiday to celebrate Ms Bristow completing a degree course in travel and tourism.
Mr Briffaut's family have set up a crowdfunding page to raise £75,000 to help pay for his care when he returns home, which has so far raised more than £32,000.
Mr Briffaut's uncle added: "The family would like to thank the Consulate for its magnificent support over the last few days.
"We are hoping that David's travel insurance will pay for him to be flown home, but after that he faces a very uncertain future.
"He has undergone surgery and is partly conscious, but in a great deal of pain.
"We have been told he may need more surgery later this week."
To make a donation visit the crowdfunding page