The name of the song sung by Marco and Debbie on Sealab 2021 was Águas de Março (Waters Of March) by Antonio Carlos Jobim and Elis Regina from their 1972 album Elis & Tom.

You can download the Sealab version from the Adult Swim web site.

The lyrics and English translation for Águas de Março can be found here.

iTunes link for Elis & Tom.

Side Note: Do a search in the iTunes Store for more covers of Águas de Março. I even found one with a Japaneses flavor to it.