Señorita Maria was a special agent for the Spanish Armada and its navy.
When Marty McFly saw her in 1697, he attempted to get to know Maria better. Later when Marty was forced onto Mac the Black's pirate ship, she revealed that she was Señorita Maria Estrada, on a mission to recover the flagship of the Spanish Navy that had been captured by the real Mac the Black. Unfortunately, the ship sank moments after the Spanish fleet was preparing to take control of it once more.
Behind the scenes
In the closing credits, it turns out that Maria Estrada also happens to be the name of the casting director for Back to the Future: The Animated Series.