manaphy will be at eathier the beach zone or the iceberg zone if you talk to wingullshe probaly will appear.
Beach zone
He is in the beach zone
go to the meadow zone and talk to bidoof him he will be asking if you need help in the beach zone
Gyarados is located next to the island in which Piplup and Wynaut are on. To get to him, you must first visit Bidoof in the Meadow Zone, do what he asks you to, then he will send his friend (a Bidoof) with you to the Beach Zone. There, you must gather lumber for him to build a bridge to Piplup and Wynaunt. After that, you will need to get more lumber for him and he will build the bridges leading you to Gyarados.
On the island where gyarados attraction is
Go to the Meeting Place and talk to Corphish
beat venasuar in meadow zone and befriend all Pokemon in meadow zone
You go talk to the guard and he let's you in
In the Beach Zone ,not that close to the Drifblim Stop
you need to build a bridge. You do this by getting help from bidoof.
If you mean how to get to Sneasel on Pokepark, you must go through the Beach Zone, and talk to Feraligator to break open the rock leading to Lapras who will take you to the Zone Sneasel is in. But first you will have to do some tasks or favors for Feraligator for him to open the rock.