In the second case, the female they were fighting over looked completely uninterested.
In fact, a few minutes after I took that photo, she hopped into the water and swam away. The males were so absorbed in the fight that they didn't even notice. It all looked very vicious, but no damage seemed to be done beyond the loss of a few breast feathers.
Eventually one admitted defeat and flew away, but by then the female was long gone. Hopefully all this fuss will have resulted in some ducklings by my next visit.