Possible Clues:

• Do some shoal searching?
• Breathing tube taking nitrogen and oxygen right through half of body's frame
• King going in to be stertorous, having left tube
• Tube for swimmer
• Breathing tube for divers
• The tube for below-surface travel
• Tube needed to be below the surface
• Tube supplying air to swimmer
• Bent tube for a swimmer's mouth
• Breathing device for swimmers