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
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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:)