The line demonstrating the use of an understatement is the following:
<em><u>BOB (still looking out—looks up): Still all right up above, though. </u></em>
Having read the previous lines, the reader gets the sense that the situation is critical, the water is rising and it has already flooded the whole lower part of the hotel. However, Bob wants to diminish the emergency of the situation by stating that the upper part still looks alright meaning that the extent of the damage is not as great as Ed says.
