A Transport Boat will appear near the end of the level. Until then, the player will have enough time to be prepared.
Unlike other zombies, the Transport Boat is invincible but it will not attack the player directly. Instead, it will release Flying Imps onto random column locations. This can be extremely dangerous as if the player does not bring Homing Thistle or any other plant that can attack backwards they will hopelessly see all of his ground plants eaten.
There are also lightning storms and Lightning Gun Zombies which can neutralize the player's plants in extreme speeds. Anthurium is advised, but it will not help against Flying Imps despite it has two heads facing different ways.
The Transport Boat might obscure some of the zombies near the edge of the player's castle, making it hard to tell how many zombies are near the castle.