Instructions
Obtain a Pokemon that can learn Surf and Dive. Examples of Pokemon than can learn both moves include Sharpedo, Carvanha and Slowbro.
Surf to the Abandoned Ship stranded on the northern portion of Route 108. The ship is located halfway through the Route.
Enter the ship and use Dive to access the lower half of the ship. Surface to enter a new room with several locked doors.
Walk along the room and press the "A" button with every step to search the immediate floor for the five keys that open the doors. The Scanner is located in the second door. Grab the Scanner and leave the Abandoned Ship.
Travel to Slateport City and talk to Captain Stern. Trade the Scanner for the Deepseascale.