In the original Plants vs. Zombies, Sea-shroom is the plant that can be placed directly on water without a Lily Pad.
The pool appears in the backyard stages, where most plants need a Lily Pad as a floating platform. Sea-shroom is an aquatic mushroom, so it occupies a water square by itself. Tangle Kelp also bypasses the Lily Pad requirement, but it is a one-use plant that drags a zombie underwater rather than serving as a conventional attacker.
Cattail is commonly confused with the answer because it is an aquatic plant, but it must be planted on a Lily Pad. Lily Pad is the platform itself, not the plant that avoids needing one. Sea-shroom is especially useful in early pool levels because it costs no sun at night, although it becomes dormant during daytime unless awakened with Coffee Bean.
The original game was released by PopCap in 2009. Its pool mechanics made plant placement part of the strategy: players had to budget for Lily Pads while defending six lanes, including two water lanes.