I think it is important that we consider the scene in which the title appears. Late in the novel, in the full throws of the hurricane, Janie and Tea Cake and others are huddled in the corner of a shack, starring wide eyed at the door in complete fear of the hurricane just outside. The humans at this point are powerless against the hurricane; their fate has been decided. The eyes are looking toward an omniscient, omnipotent God to share their fate with them.
Oprah Winfrey's production of "Their Eyes Were Watching God" shows Janie several times lying in water gazing at the sky and horizon. This interpretation of the title is one that suggests that Janie and other women who were free spirits looked beyond the confines of the role of women, especially the repressive role of African-American women in the early part of the twentieth century.
Perhaps the God is the God of Nature Whose dominion is expansive and without human boundaries. Janie searches for more meaning in her life, more fulfillment, places beyond boundaries.
What an interesting and insightful question. In Their Eyes Were Watching God there is such a dichotomy between the graceful poetic language and the elements of harsh, ugly realities. Janie's God is clearly a God of both. When she is in the worst despair, relief is provided. Just as they (she) are watching Him to discover their fate, He is watching over them.