The man initially swam from the Dumfriesshire side of the Solway Firth to Cumbria on Sunday afternoon, a distance of about eight miles.
And his arrival at Silloth caused a flurry of concern from onlookers, who contacted the local lifeboat station and Maryport Coastguard.
A crew headed out to meet him and check he was ok and to see if needed their assistance to get back.
Explaining what happened next, Silloth lifeboat spokeswoman Julie Emary said: “He had one of those orange flotation bags with him and that’s what people spotted in the water.
“We had lots of people contacting us concerned there was someone in the water. A lifeboat went out to him and spoke to him and he assured them he was perfectly fine and had swum here and would swim back.
“At this stage, the lifeboat decided he knew exactly what he was doing and was capable, but they decided to shadow him.
“However, the tide was out so they could not actually physically escort him back to Scotland. The last time he was seen was running across the sandbanks to Scotland.”
Julie stressed the dangers of swimming in the Solway, even for experienced and strong swimmers.
She said: “This man had no support crew and no-one with him and had not contacted anybody.
“He said he had done a recce in the winter and that people were waiting for him at Caerlaverock. It would have been better to have had someone with him.
“We would not advise others to do this because the tides are so unpredictable and the currents can easily take you out without you realising how dangerous it is.
“We have had a lot of callouts this year.”
Keen to know if the man got back safely, she added: “We asked him to contact the lifeboat station or the coastguard to let us know he was back but he has not.”
* Are you the mystery swimmer, or do you know his identity? Contact [email protected]