Sott'acqua literally means "underwater". It's a less common, more evocative phrase than simply saying "in the water", adding a sense of depth and immersion.
In the song, Marco Mengoni sings about spending hours "con la testa sott'acqua" (with his head underwater) to preserve the silence of the sea. This imagery beautifully conveys a desire for peace and introspection, making it a memorable and interesting word choice.
Marco Mengoni looks back at who he used to be before this relationship. He remembers having more personality and confidence, and even liked simple things like holding his head underwater just to feel the quiet of the sea.
Now he is with someone who keeps him at a distance. She would rather watch TV than be close to him, and he compares the wall around her heart to the Berlin Wall. He studied Spanish, but the only thing he ends up saying is "hola, hola" - the song uses that one small word to show how little real conversation is left between them.
"Hola" moves between missing his old self and feeling shut out by someone who never really lets him in.