You don't need the boat ticket in Pokemon Platinum because it's the ticket for Luxury Liner SS Anne, which is not in the Sinnoh region.
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There is no use for the SS Ticket in Pokémon Platinum.
beat the elite 4 then go back to the boat
The piplup boat is during the Pokemon parade at wifi(the bottom of poke center)
after getting the ticket, talk to the boatmaster at the docks
after u defeat Pokemon lege u get ticket then fly to olivine then take the boat 2 kanto