The Snorkel Zombie.

Wherever there's water, there's bound to be Snorkel Zombies. They jump into the pool and swim near the bottom, out of range of all shooters excluding lobbed-shot plants. He has to come up to eat, though, giving your plants an opportunity to attack.

Health: Low

Strengths: Out of range of many plants

Weaknesses: Lobbed-shot plants, Tangle Kelp