I'm guessing the arena is probably filled to at least the humans waist height if not higher with water. In the water dolphins are fast, nimble, able to break bones with wallops and cut up our skin and muscle with their razorteeth.
Humans in water may as well be injured or handicapped in comparison. we are clumsy and can only remain under the water for around a minute and even that long is hard to do unless you are in a calm zen-like state.
Common bottlenose dolphins average weight is anywhere from 150 to 650 kg , yes you read right 650kg. the average dolphin can reach over half a tonne.
At speeds of around 30km/hour and all they have to do is keep our heads under water to kill us. I'm afraid even with basic tools the dolphins got us beat.