Submarino literally means "submarine". It's a fantastic and unusual word to find in a love song!
In the lyrics, Manuel Medrano sings, "voy con toda la fuerza de un submarino a conquistar a esa dama" (I'm going with all the strength of a submarine to conquer that lady). He uses this powerful metaphor to describe his deep, unstoppable determination and force as he goes after the person he loves. It creates a vivid and memorable image of love's power.
Manuel Medrano wrote a romantic pop track about the exact moment you decide to stop pretending you are "just friends." Before this person showed up, he describes himself wandering aimlessly: "Voy por el mundo solo y sin amigos / Voy dando tantas vueltas sin ningún sentido" ("I go through the world alone and without friends / I wander around with no direction at all"). Meeting her flips a switch. Suddenly he feels powered up, comparing his romantic drive to heavy naval machinery: "voy con toda la fuerza de un submarino" ("I am going with all the strength of a submarine"). It is a funny, slightly dramatic comparison, but it shows he means business.
The real heart of the track is how bold he gets about ditching sorrow. He wants to dress up, walk out in public, and tell anyone listening, "no somos amigos" ("we are not friends"). He even issues an order straight to misery itself: "Decirle a la tristeza / Que no se cruce en mi camino" ("Tell sadness / Not to cross my path"). Medrano credits her with giving him "mucha más vida que el vino" ("much more life than wine"), turning cold, aimless days into late-night daydreams of watching stars over the ocean.