From the Super Mario Wiki
The Peapod is a sailing ship owned by Captain Abidab. For some reason, the Peapod resembles a large, old boot with a mast coming out of it.
In The Super Mario Bros. Super Show! episode "20,000 Koopas Under the Sea", wanting to hunt the sea monster attacking Happy Harbor, Captain Abidab had Mario, Luigi, Toad and Princess Toadstool act as the crew of the Peapod; a job which Mario, Luigi and Toad were somewhat terrible at.
After the search for the sea monster came up fruitless after a while, Captain Abidab, Mario, Luigi, Toad and Princess Toadstool left the Peapod to search underwater for the sea monster; which they found and were attacked by, in a skirmish which resulted in Mario, Luigi and Princess Toadstool being eaten. After narrowly managing to escape the sea monster, Captain Abidab and Toad reached the surface, where they saw the sea monster briefly attack the Peapod.
Apparently damaged by the sea monster, the Peapod now required Toad to bail water out of it; as he bailed water, Toad saw a bottle floating nearby and, using his bailing bucket, reeled it in. Taking a message out this bottle Captain Abidab read it and found out it was from Mario and that Koopa Nemo was planning to attack Happy Harbor in a submarine, which was what the sea monster really was. After reading this, Captain Abidab and Toad steered the Peapod through a shortcut to Happy Harbor.
After Koopa Nemo and his submarine, the Koopilus, were defeated, Mario and Luigi, who landed on the Peapod during the battle, were cheered at by Toad, Princess Toadstool, Captain Abidab and the grateful inhabitants of Happy Harbor.