Possible Clues:

• The Rhine flows into it
• Savings primarily invested in another fresh source of oil and gas
• Don runs into this, bearing witness for hearing
• The Hague's water
• Part of the Atlantic Ocean (2 words)
• Waters west of Denmark
• Waters off the British Isles
• Water between the UK and mainland Europe
• Oil-bearing region on earth's being developed
• Heart's no different -- it's this water