Possible Clues:

• Boat that clears the bottom of a river
• Vessel used for deepening a channel
• Canal-digging machine
• Boat for deepening waterways
• Boat used to scoop out mud etc. from the bottom of a river
• Boat used in scooping out mud, etc., from the bottom of a river
• Boat removing mud
• Vessel, one making gradual progress on way back
• River-scooping boat
• Vessel for dragging the bottom