First, play through the bonus quest. When you do, you'll get a deep-sea diving suit that lets you stay underwater for much longer than usual. Next, restart the Island while you're still wearing the diving suit.
Now, when you start playing the primary quest again, you'll have way more oxygen, and can spend more time exploring the depths! You can even play a little ditty on the piano, as in the picture above.
There's always more to explore on Poptropica, so get out there and dive deep!
CAPTAIN CRAWFISH