This was the intended entrant for the 1927 Schneider Trophy race, but it wasn't ready in time. Had it won then the USA would have gained permanent possession of the trophy, denying the Supermarine S.5 which won it outright for the UK.
The man standing on the left hand float is Al Williams who was a close friend of Ernst Udet. He became the first American to fly a Bf 109 on 15th July 1938 when he and Udet visited the Fiesler works at Kassel.
He was impressed saying:
"I must say, however, the Messerschmitt Me 109 is the finest airplane I have ever flown"