This is how you can walk underwater. You can only do it while the flood comes (You need to finish mission 7 in each level) First, swim to where the water fall is. Next, go to the log inbetween the pipe to go to the hotel on a roof, and the waterfall. You will see 2 logs. Got to the one near the hotel pipe. Next, go under the middle of the log. Now, keep pressing the A button under the log. Keep doing it until you either somehow fall under the ground and die, or walk underwater without losing air. If you fall under the ground, keep trying.