Well you can get either Deep Sea Tooth or Deep Sea Scale from Cpt.Stern when you retrieve the scanner from the old abandoned ship. You can see the ship when you're traveling wih Mr.Briney from Dewford to Slateport. when you get surf after deafeating your dad, go there. you nned dive and then find the room its kinda hard 2 find. then when you get the scanner, go 2 capt. stern he will say he needs it and he will give you a deepseatooth or scale for it. the tooth turns into huntail, and the scale is gorybass or somthing.
You have to make clampearl hold the item and trade it to make the clampearl evolve.