High Tide!

High Tide! is a special mission in which you must complete rounds 20 to 80 while the game area becomes increasingly covered with water (causing the terrain type to change to underwater). In this mission, the game area begins with a line of water at the bottom of the screen. However, after every five rounds, the water extends farther onto the land, until eventually it is covered with water. Any towers that get covered by water will take 10 damage every round, until they are given scuba gear. If you complete the mission, you get $350 Monkey Money.