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