The player has to deal with Snorkel Zombie, which absorbs ten normal damage shots before dying and currently avoids all attacks unless it surfaces to eat a plant.
At the same time, pool lanes are generally weaker than grass lanes.
Plant Sunflowers first and collect as much sun as you can in the beginning of the level. Like most daytime levels, plant Potato Mines to kill the first few zombies and to collect sun.
The fourth zombie that comes is the Conehead Zombie. They will continue to appear. Plant Repeaters on every grass lane while starting to plant Lily Pads and Repeaters on pool lanes.
To make the level easier and end faster, plant as many Repeaters as you can evenly on every lane.
As the walkthrough suggests, plant Wall-nuts, Squash, or other really inexpensive plants on Lily Pads on the fourth or fifth column from the left to easily fight off Snorkel Zombies, but also plant repeaters on the pool lane incase the other plants don't kill all the zombies.