Cantarolar means 'to hum' or 'to sing softly'. It's a charming and less common verb than 'cantar' (to sing), suggesting a more gentle and spontaneous musical expression.
In "Fundo Do Mar," the lyrics mention "Uma baleia a cantarolar" (A whale humming/singing softly). This word paints a lovely image of a whale making music in a relaxed and joyful way, perfectly fitting the song's whimsical underwater theme.
The whole song is a list of what a child would see if they could breathe underwater. A whale hums — "Uma baleia a cantarolar" (a whale humming) — and what follows it is the song itself, "A música que dá o tom" (the music that sets the tone). Colour arrives doubled, the way small children say a thing twice to mean it: "Azul azulado, verde esverdeado" (bluish blue, greenish green). The sea tastes of salt, turtles dance ballet, and the fish turn up in three sizes at once — "Peixes, peixinhos, peixões a nadar" (fish, little fish, big fish swimming) — before they line up into a heart.
Then it stops describing and starts asking. "A maré é mansa eu quero ver" (the tide is gentle, I want to see) sets up a question the song never answers: "Quantos dentes tem o tubarão?" (how many teeth does the shark have?). It is a counting prompt dressed as a chorus, and the verse repeats in full so a child gets a second run at it.