Tangle Kelp

The Tangle Kelp is a single use, instantly armed aquatic instant kill,  dragging any zombie unlucky enough to cross its path into the watery depths of the pool but unfortunately, it can only kill one zombie. When it kills a zombie it disappears into the abyss of the pool. It is based on kelp, an aquatic algae with long, stringy leaves that have been known to tangle small fish.

Suburban Almanac Entry
Tangle Kelp

Tangle Kelp are aquatic plants that pull the first zombie that nears them underwater.

Damage: massive

Usage: single use, on contact

Must be planted in water

"I'm totally invisible," Tangle Kelp thinks to himself. "I'll hide here just below the surface and nobody will see me." His friends tell him they can see him perfectly well, but he'll never change.

Cost: 25

Recharge: slow

Overview
Tangle Kelp is an aquatic plant. It is placed on the pool and kills the first zombie that comes into contact with it. After killing the zombie, it disappears to the water and it never comes out again.

Strategy
With such a low cost, Tangle Kelp makes an ideal early defense in Pool lanes. Just don't get lulled into failing to build up your pool defenses, because the slow recharge prevents Tangle Kelp from being readily re-used.

They also defeat Dolphin Rider Zombies whole instead of being jumped over, so these may be more useful than just placing a Lily Pad in front of it. Furthermore, they are useful in defeating Snorkel Zombies, as they can drown them without having them come up. However, their slow recharge may prevent them from drowning every Dolphin Rider Zombie or Snorkel Zombie, so it's wise to prepare backup plans for them just in case. Squash makes a good backup alternative as it can eliminate multiple Zombies if they are grouped closely enough.

Trivia

 * The zombie that was pulled underwater by a Tangle Kelp does not die because the Snorkel Zombie's Almanac Entry states that zombies do not need to breathe. Therefore, it is unknown what the Tangle Kelp does to the zombie it pulls down.
 * The Tangle Kelp and the Tall-nut are the only plants that the Dolphin Rider Zombie can't jump over.
 * Tangle Kelp has a similar appearance to the Pokemon Tangela.
 * The Tangle Kelp, Spikeweed, Spikerock and Coffee Bean are the only plants that zombies will never attempt to eat.
 * Along with the Sea-shroom, the Tangle Kelp is one of two offensive plants that don't require Lily Pads to be in the pool.
 * The Tangle Kelp, the Sea-shroom, the Grave Buster and the Cob Cannon are the only plants that cannot have Pumpkins placed around them, excluding the Pumpkin itself.
 * The Tangle Kelp, the Chomper, the Potato Mine, the Hypno-shroom and the Squash are the only instant kills that are used on contact with the enemy.
 * The Tangle Kelp, the Chomper and the Squash are the only instant kills that do not explode.
 * On the Plants vs. Zombies website, the Tangle Kelp is seen on land, instead of in the water.
 * In the Almanac Entry for the Tangle Kelp, it says the Tangle Kelp thinks Zombies and plants cannot see him. This may be true because zombies never attempt to eat Tangle Kelps and Dolphin Rider Zombies will ride past them instead of jumping over.
 * Tangle Kelp is one of the twelve plants that appear in the seed selection screen after you get the Grave Buster on the online version with the others being Ice-shroom, Lily Pad, Potato Mine, Threepeater, Doom-shroom, Jalapeno, Spikeweed, Torchwood, Tall-nut, Cactus and Blover.
 * The Tangle Kelp can be considered the aquatic version of the Potato Mine except it doesn't need time to arm and is not area of effect. However it costs the same amount of sun and has the exact same recharge time.
 * In ZomBotany 2, one Tangle Kelp can kill a Tall-nut Zombie.
 * When tangling a Dolphin Rider Zombie, you can actually see some leftover space where the zombie is supposed to be tangled, possibly because the Dolphin Rider Zombie is smaller than a normal zombie.