Dave's mold colonies covers all plantable squares, forcing players to use Lily Pad as the tide owns the rest. This also makes all plants on the frontline vulnerable to Imp Mermaid Zombie attacks.
This level is a more difficult version of Big Wave Beach - Day 7, which had no shore available, but the whole lawn was usable. With 10 less spaces in Day 9, you have less options and are closer to the zombies.
This level is also difficult due to zombies entering the lawn in hordes early on in the level. Two zombies enter into the lawn at the start of the level, instead of the usual amount of one.
The small number next to the zombie icon means that zombie will always appear in that lane, with 1 being the topmost lane and 5 being the bottommost one.
Collect sun and place Twin Sunflowers on the leftmost plantable column.
Place Tangle Kelp in the column nearest to the Twin Sunflowers. This allows the player to delay time, overcoming the Tangle Kelp's slow recharge so it can be used to kill both the first two zombies and also to collect more sun.
Start placing a Wall-nut or Tall-nut in each row near the Twin Sunflowers such a way that every row has some form of defense.
Use Tangle Kelp, Ghost Pepper and Cherry Bomb as deemed fit, reserving Cherry Bombs as a last resort. Preferably, Tangle Kelp should be used on lone zombies or to clear out zombies that were not killed by Ghost Pepper while Ghost Pepper and Cherry Bombs should be used to clear out crowds of zombies or if strong zombies are present.
Continue building up the defense as the Wall-nuts and Tall-nuts get eaten. Eventually, it would be easy to have a column of Wall-nuts and Tall-nuts adjacent to the Twin Sunflowers. This will prevent the ambushes resulting in devastating damage. With constant replacing of eaten Wall-nuts and Tall-nuts, this level should be a breeze from here.
Sweet Potato (if you have bought it or you have the eighth slot)
Place Twin Sunflower on the third column.
Use tangle kelp on the first zombie comes until the end of first flag or Homing Thistles if you had enough sun.
Place Snapdragons on the fourth column, to attack Pompadour Zombies and Bikini Zombies variant and for the Imp Mermaid.
Until the second, third, fourth flag, use the same technique, if a plant (e.g. Snapdragon) was eaten, use Jalapeno or Cherry Bomb or the Sweet Potato to change their lanes, then bang it up with a Jalapeno. When the final wave comes, and with your loaded weapons (Cherry Bomb or/and Jalapeno), use them as the Sweet Potato, then put Sweet Potato in the middle if the zombie on the first lane and the fifth lane were dead, then, use a plant food on a Snapdragon and bang them all.
This strategy is only used for the ones who bought Jalapeno, Homing Thistle, Sweet Potato and the 8th slot.