Should have googled properly before posting the question. As it turns out, the creative decision was actually a workaround to a date problem. Fincher actually wanted to shoot the whole scene in Henley Royal Regatta but it was available for a very limited time.
So Fincher decided to shoot the closeups in Eaton on a man made lake which didn’t look anything like Henley. In his own words -
We could only shoot 3 races at the Henley Royal Regatta; We had to shoot 4 days of boat inserts in Eton. The only way to make the date for release was to make the backgrounds as soft as humanly possible. I decided it might be more “subjective” if the world around the races fell away in focus, leaving the rowers to move into and out of planes of focus to accentuate their piston-like effort.
Huh!! Kind of anticlimactic.
Here is the link to the article from where the official statement was copied.