I call it subtle, because up to the last episode, the show had stuck to subtext rather than text. To me the message was pretty clear, but my friend didn't realize what the boat scene was about until I pointed it out to her. Then she pointed out that the kids kept referring to Sam as Uncle Sam, and now I felt dumb for not picking up on THAT.
But I like this way of presenting a message about current social issues. Just as long as they don't have a character turn to the camera and give a lecture. I feel a show is more powerful when it makes you think about its message, rather than telling you what its message is.