After roughly a minute or so of swimming underwater, your character will begin to lose oxygen. As you begin to drown, your vision will begin to blur and you will slowly take damage. However, there is a way to survive longer underwater, and believe it or not, it revolves bananas.
Should you a banana while your character is slowly drowning underwater, your health meter will actually be replenished, giving you even more precious seconds to swim. Since you can hold up to five bananas at any given time - even more so in Shipwrecks - you will be able to stay underwater for much longer than possible. Just keep eating and swimming!