Despite the event at Odaiba Marine Park, part of the "Ready Steady Tokyo" series of test runs, starting at 7am, the temperature was already over 30 degrees Celsius in the Japanese capital city.

A total of 57 people have died across Japan as a result of a heatwave between July 29 and August 4 and the country's main broadcaster NHK reports another 18,347 people have been taken to hospital, with temperatures remaining above 31 degrees Celsius since July 24.

"That was the warmest race I've ever done," three-time Olympic medallist Oussama Mellouli from Tunisia told Agence-France Presse (AFP).

"It felt good for the first 2km then I got super overheated."

FINA rules state that athletes cannot race in open water swimming events when the water temperature is higher than 31 degrees Celsius.

FINA executive director Cornel Marculescu promised they would continue to examine how to best ensure athlete well-being, and said the start time in the Olympic event could be moved even earlier.

"Based on this information, we will decide the time the event will start," he said. 

"Could be 5am, could be 5:30am, can be 6am, can be 6:30am – depends on the water temperature.

"Working with a specialised company like we are going to do here in Tokyo, we will have the right information to take the right decision."

"The water temperature was high, so I'm a bit concerned about that," Japanese swimmer Yumi Kida told AFP. 

She also told reporters that the water was "a little stinky, and the clarity was not very good so I really want to improve the quality".

Water quality at Odaiba Marine Park, which will also host the swimming leg of the Olympic triathlon, had previously been a concern after worries it would not meet FINA's health standards.

FINA and the International Triathlon Union had raised concerns in 2017 after test results showed levels of E. Coli up to 20 times above the accepted limit, and faecal coliform bacteria seven times higher than the permitted levels.

Those worries have been allayed in recent months with the introduction of underwater filters, and FINA Medical Committee member David Gerrard told reporters that disease was now a lesser worry.

"What we have had are readings from the last month, daily readings that have given us very clear indications of the water quality, which has been good," Gerrard said. 

A total of 35 athletes, 22 men and 13 women, took part in the test event.

Both races were contested over five kilometres, half the Olympic distance of 10km.

Tokyo 2020 told insidethegames that the event was for operational testing purposes, and therefore no results were issued.