i guess alex is half right..
anyways go to teh abandoned ship and get the scanner
go back and find the professor in the place where you can ride that boat
(lol i havent played this in so long)
the professor should be there adn you give him the scanner and you get your choice of deepsea scale/tooth
i believe its the dock in teh city south of mawvile (electric gym)
sorry i dont remember much of the 3rd generation XD