first you need the Dive HM,then in the room by the "storage room"-->(U know the one thats locked!)just below that room and to the right ull find a room with water in it,then surf on it and then dive,go through the entire path and at the end of the new passage u'll find the scanner.Bring the scanner to that one nerd upstairs thats asking about it,he'll tell u to bring it to Capt. Stern,and Stern will be in(I think) the Slateport Harbor.Bring it to him and he'll give u either(only one...I know bummer only one to choose and its impossible to get the other without trading with someone else or starting a new game)DeepSea Tooth or the DeepSea Scale,which will let U evolve into either Huntail with the DeepSea Tooth or Gorebyss with the DeepSea Scale.
Hope I helped!
~Engel der Bosheit
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