My friend has a business descaling ships with his own device. The device consists of a "pressure washer" "robot" that sticks to the side of the ship with combination of magnets and suction.
The motor+pump for "pressure washer" is about the size of minivan and resides on it own trailer. The "pressure washer" (if you made it a single nozzle) will cut through an inch of steel. The magnets are arranged in a pattern called a Halbach Array that concentrate the magnet force on one side of the array.