Related Clues:

• Hero sandwich
• Deli sandwich
• U-boat
• U-boat, e.g.
• Part of the game
• Underwater craft
• Underwater vessel
• By the sound of it, you get a boat for this
• See 20
• See 6
• At sea as number one in warship
• Underwater (vessel)
• … underwater craft
• Irrational number is a means to go deeper
• Three units, in 56-Across