Possible Clues:

• Submerged
• Mortgaged to the hilt and then some ... and where it's at
• Being in drink here admitted him to the 19 1
• Like the fish hidden in 20-, 34-, and 51-Across, if you look up a row?
• Prepare your flowers for a dry spell below the surface
• Neglect your garden plants? Not on the surface
• There it's never non-U in the navy
• Either leave dry or inundated
• Where there could be a submarine
• Masks are worn here