I've watched it a few times and I reckon it was a publicity stunt.
If you watch it make sure you have the sound on. There are a couple of things, there is no splashing from the blades spinning when he pulls it into the water and you can hear the motor still running after the mower is completely submerged.
Its the middle of the holiday season here so what better way to get people to come to the park, you have to pay entry to the park.
http://www.youtube.com/watch?v=G_ABA-xceo0