Maravilhosamente means "wonderfully" or "marvelously". It's an adverb derived from the word "maravilha" (wonder/marvel), and its length and sound make it quite distinctive.
In the song, it's used to describe the clarity of the water in the "lindo lago do amor" (beautiful lake of love), painting a vivid and enchanting picture. It's a word that evokes a strong sense of beauty and awe, making it memorable and enticing.
Alexandre Pires turns a bedtime-story cast of nature characters loose on love. A bem-te-vi bird, a thrush, the moon, the sun, the rain, the wind, an eagle, and a frog each play a small part, almost like a fable read aloud to a child.
The frog gives the answer everyone else is chasing: he took a bath in a wonderfully clear lake, and it turns out to be the lake of love. The song circles back to that image again and again, repeating it like a chorus a child could sing along to.
It plays as a light, whimsical song rather than a heavy love ballad, using the small everyday signs of nature as a stand-in for love itself.