I speak Spanish, I searched "Canciones de Marineros"(Literally 'Sailor Songs') There was only a few things. I eventually found "Cancion de Trabajo"(Work songs)It mentions sailor songs, but is not exclusive to Spain, so it talks a lot about U.S. Slave songs. My completely honest guess would be that they sang religious Catholic songs due to the fact they were often transporting Franciscan Monks to Mexico, South America and California. That is my best and only guess.