Mario's torpedo-armed submarine in Super Mario Land. He only gets to use it in the first Shooting Area (World 2-3).
Just like Mario's clothes, the Marine Pop is form-fitting: it gets larger when Mario becomes Super Mario and vice versa.
These are some of the underwater enemies Mario can annihilate with his sub:
Yurarin
Yurarin Boo
Torion
Gunion
Dragonzamasu
Here's an ASCII picture of regular Mario puttering about in his Marine Pop: ########
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