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"A tiny island in the blue sea." At Dewford you can first pick up a Silk Scarf in one of the houses. It can give some of your normal type Pokemon like Whismur or Skitty a nice boost! You can also grab an Old Rod from a fisherman here as well. Dewford Hall is building where you can be something of a modern day influencer and say something "trendy" to have it echo out throughout Hoenn (in other words it's a big waste of time if you ask me). It can change where Feebas tiles spawn though and you CAN get TM Sludge Bomb here after you earn your fifth badge from your dad.
Putting that old rod to use let's you catch Magikarp and Tentacool. Magikarp of course already has a good reputation in its evolution Gyarados, but Tentacool, as common as it may be like Wingull, is actually a fantastic Pokémon itself with great Speed, Special Defense, and Special Attack. It evolves into Tentacruel at level 30.
Up north we come across Granite Cave, inside residing some very nifty Pokémon!