A man in his 30s and a woman, in her 20s, were found under the water by workers at Down Hall Country House Hotel, Hatfield Heath, Essex.
Ambulance crews and staff from the hotel, on the Essex-Hertfordshire border, were unable to revive the pair on Saturday evening.
Police said there was no evidence of foul play. Post-mortem examinations are due to be carried out on Monday.
Ch Insp Nick Lee, from Essex Police, said they are treating the deaths of the pair, who are said to be from the London area, as unexplained.
They had been staying together at the hotel, which was hosting two wedding parties this weekend.
Guest Louise Saunders told the BBC an ambulance and two paramedic cars arrived at the hotel as she was leaving to go for dinner.
She said about three hours later police were "everywhere" and were taking down the names and numbers of guests.
The windows of the building housing the swimming pool had been blacked out with plastic sheets, she added.
The hotel's manager, Chris Falcus, said he was "devastated by this tragic incident".
"As soon as the alarm was raised two of our team dived into the pool and did everything they could to try to save them," Mr Falcus said.
"We are co-operating fully with the police investigation."
The hotel describes itself as one of England's most established country house hotels.
Some of parts of the building date back to 1322 and it sits in 110 acres of surrounding woodland, parkland and landscaped gardens.
Two people have died in a swimming pool at a luxury country hotel.