u need to complete the whole Sinnoh pokedex, then go to professor rowan and hell know that u completed it, professor oak upgrades ur pokedex to national. then once u go back to that area the men blocking ur way to route 230 will be gone. if u played through the game by fighting every trainer and not skipping one trainer then u have seen every Pokemon. one tip i can give u about a Pokemon u haven't seen is manaphy, so just go the Pokemon Mansion in route 212 and go to the room with the owner in there, there will be a book, press a to view it and u will see a picture of manaphy, that counts as seeing the Pokemon, till then u shoud be done with completing the Sinnoh dex.
good luck
You can go fish at route 223 and route 230 with a super rod and eventually you'll find one
you get the National Dex.
You have to visit the battle tower thingy first, then they let you past
There are a few places you can find a dewgong in on Pokemon Platinum version. You can find him on Victory road, route 226, and route 230. If you catch a Seel you can also get Dewgong by leveling Seel up to level 34.
complete the sinnoh dex.hope this helps:)
route 223 or route 230
if you mean in Pokemon diamond/pearl/platinum you need to get the national dex!
route 223 - 230 super rod
beat the elite four then follow route 230 and route 225
Go north of Resort area, and south of route 228
You can go fish at route 223 and route 230 with a super rod and eventually you'll find one
Pastoria city with the super rod and route 230 with super rod
you get the National Dex.
You can hatch the egg given to you by Cynthia or find them on Route 230 with the Poké Radar.
You have to visit the battle tower thingy first, then they let you past
You can find them at these places: Route 212 South, Route 213, Route 222, Route 223, Route 224, Route 230, Pastoria City, Sunyshore City, Pokemon League. (Fish with Good Rod.)
There are a few places you can find a dewgong in on Pokemon Platinum version. You can find him on Victory road, route 226, and route 230. If you catch a Seel you can also get Dewgong by leveling Seel up to level 34.