Throughout most of the level, there is mostly water and not much solid ground, and the ceiling is almost entirely made up of spikes. All of the terrain and the background seems to be made from the insides of the Whale itself. This level is the only appearance of the Fish Bone enemies, which act like normal bird enemies. Since the level is an even number, the level ends with a boss fight with the final shrimp. As with all other boss fight levels, Jim can be seen floating away before the boss fight.
Oddly, a similar level appears in Super Mario Land 2: The 6 Golden Coins, where the level takes place entirely inside a whale. It should also be noted that both Balloon Kid and Super Mario Land 2 were worked on by Nintendo R&D1.
There is also a level which takes place inside Jabu Jabu, a large whale-like character, in The Legend of Zelda: Ocarina of Time.