Possible Clues:

• Deployment of the sonar in marine area
• UK's source of oil, gas
• It's not land to our East, and not the area round South
• You've walked way too far and get to this body of water near Britain. Search the shallow area, see 24-down. Go deep, see 63-across.
• German Ocean
• UK source of gas
• Oil production area
• Finally exhausts another developed source of oil
• UK source of natural gas
• It is mainly to the east of Britain