Possible Clues:

• Skin diver's tube
• Thousand attending tedious event left in tube
• Swimmer's aid -- something to help one see coming up round Orkney -- not half
• Inspiring device
• Diving apparatus soon empty round river with a lot of kelp
• Wild elk's holding neither breathing device
• Diver's equipment
• Explore coral reefs, perhaps
• Reef explorer's tube
• Left on Orkneys, losing last of very new breathing gear