Just why do humpback whales sing? That's the question that has marine biologist Nate Quinn and his crew poking, charting, recording and photographing very big, wet, gray marine mammals. That is, until the extraordinary day when a whale lifts its tail into the air to display a cryptic message spelled out in foot-high letters: BITE ME. Trouble is, Nate's beginning to wonder if he hasn't spent just a little too much time in the sun. 'Cause no one else saw a thing- not his longtime partner, Clay De