One Tartan Army member became a ball boy in the Serpentine and had to be fished out by a rescue boat.
It comes as football fans descended upon London for Scotland's showdown against England at Wembley tomorrow.
The fan had swam after a ball that had bounced into the water but the ball moved ever further away.
He was pulled out as he struggled in the middle of the lake.
Police warned him not to repeat the antics and helped him warm up before going back to the party.
One bobby said: “He was spoken to and allowed to go back to the party after we were assured of his safety.
“He was breaking quite a few by-laws by swimming in the Serpentine but he was not put under arrest.”
Tomorrow will mark Scotland's second game of the Euros 2020 after competing against the Czech Republic on Monday 14 June.
London Mayor Sadiq Khan told Scots not to travel down unless they have tickets or a safe place to watch from.
The Tartan Army will also be required to provide proof of vaccination or alternatively, a negative coronavirus test.