The Old man is the owner of the Wailord-shaped boat.
Pokémon the Series: Diamond and Pearl
When the boat leaves, the old man uses his Mantine to help rescue the Pokémon that were stuck on the boat and from the boat hitting a rock. The Old Man's Mantine took Ash and Dawn to the Wailord shaped boat that their Pokémon were trapped on. It then rescued them by destroying a rock in the path of the boat.