Acquiring the Quest
Go to Slateport City and visit the Oceanic Museum. It costs $50 to enter. You'll battle several Team Aqua Grunts once you're inside. Defeating them will award you with TM46 ("Thief") and allow you to speak to the people inside. One of the characters you meet is Captain Stern, who will mention an abandoned ship that contains an advanced sea scanner. Captain Stern is the only character on the second floor of the Oceanic Museum. He has brown hair, glasses and is standing in front of a model ship in the center of the room.
Getting the Scanner
Use a Pokemon who knows "Surf" to surf from Slateport City to the Abandoned Ship. After you speak with Captain Stern, it will be on your map. Once you're there, use your Pokemon's "Dive" skill to access the underwater portion of the ship. Down there, you will find five locked doors. To open them, walk over to the shiny spot on the floor and press the "A" button. This will give you either a key or a piece of trash. If it is a key, you may enter the first locked room. If it is trash, a new shiny spot will appear. Keep examining the shiny spots until you receive a key and can enter the first locked room. Once you're inside the first locked room, keep examining the shiny spots until you get the key for the second room. Continue finding keys in the remaining rooms. Once you access the final room, you will find the Scanner rather than a key. Examining it will reveal that it is too complex for your character to operate.
DeepSeaTooth
Take the Scanner back to Slateport City and give it to Captain Stern at the Oceanic Museum. He will offer you one of two items in trade. One of the items is the DeepSeaTooth, which has two effects. While it is in your inventory, it will double the effectiveness of your Clamperl's special attack. If you trade it or sell it, it will cause Clamperl to evolve into Huntail, an eel-like Pokemon with a much higher attack rating.
DeepSeaScale
The other item Captain Stern offers for the Scanner is the DeepSeaScale. The DeepSeaScale operates much like the DeepSeaTooth. When it's held in your inventory, Clamperl's special defense is doubly effective. When it is traded or sold, Clamperl will evolve into Gorebyss, an eel-like Pokemon that has a much higher defensive rating.