The player had to deal with the Dolphin Rider Zombie, the faster pool version of Pole Vaulting Zombie. After Dolphin Rider Zombie jumped over one plant, it lost its dolphin and moved like a regular Zombie, similarly to Pole Vaulting Zombie.
The objective also increased the difficulty a little bit.
Torchwood (only if you choose Repeater, and have nine seed slots)
Plant three rows of Sunflowers with Potato Mines to kill the beginning zombies. Once you have enough sun, plant Repeaters or Snow Peas on every lane. Use two of them on pool lanes.
Plant Squash in the pool lanes so that the Ducky Conehead Zombies die before reaching them and the Dolphin Rider Zombies will get dragged down by them.
Plant at least two columns of Wall-nuts. In the pool lanes, and Spikeweeds in every land spot not taken to defend the expensive Repeaters.
In the final wave, two Dolphin Rider Zombies and the three Conehead Zombies (that emerges from the pool) are the biggest threats. Make sure you have enough Repeaters and Squash to deal with them.