Possible Clues:

• Measurer of depth, thus submerged by a nymph
• Depth measurer that's safer under City house?
• Sea depth measurer
• Nymph's very down -- that shows depth
• Means of determining depth at sea
• Aid in deep diving
• Sonar
• Device that can map seabed
• Lovelorn maiden, very down, used to hit rock-bottom
• City house with greater security for navigation device