Because bronze is a copper alloy, the surface copper oxidises, giving it the patina seen on the watch. The green bits are all the results of the oxidisation of the copper. The Statue of Liberty is clad in copper, hence its green colour.
Exposure to seawater – Laurent went swimming and kite surfing – will accelerate the surface changes and also produce a much more pronounced patina that you see here.
But the patina actually protects the alloy underneath as it’s a surface change so it is a good thing. That’s why antique bronze objects are green and last so long.