I think he wanted to chase them but either decided the water was too cold or they were too far out in the pond. Or both.
He can swim that far out, though. He's done it before. In fact, he usually only swims a long way if he's swimming after geese or swimming with another dog. When he's by himself, he typically just takes quick dips and doesn't swim too far.
It was chilly today, though. I think he decided it wasn't worth swimming all that way to get to the geese. He decided it was better just to run very fast.