Briefly, the boat is sinking. One passenger, our hero (heroine?), has a life raft. The other passengers make trivial objections ("I don't like the color") and detach the raft as an act of protest. Of course, they all drown, and blame the hero for not offering "a more inspiring raft." The obvious metaphor is that the boat is our Republican-led government, our hero represents the mainstream "centrist" Democratic party, and the inane passengers are progressives.
The first problem is that as bad as our political situation is, a sinking boat is a terrible metaphor. A country is not a boat we can just abandon, at least not en masse.
The second problem is that it's egregiously insulting to trivialize the objections of those of us fail to support the mainstream Democratic party. While I don't object to insults per se, insults do not persuade, they marginalize. An insult says, "I do not care about your opinions; they are not worthy even of rebuttal." The message is that mainstream centrist Democrats such as Myers simply do not care about the objectives of progressives: our political situation is simply too dire to permit dissent. (Of course, it's always too dire, and when our situation isn't so dire, when the boat isn't sinking, it's just utopian foolishness to worry about the life boat, n'est ce pas?) The progressive and radical objections are more like, "The life raft will fit only 10% of the people (including economically privileged people like Myers) and you expect the rest of us to get in the water and push you to safety."
But the big problem, the real problem is just this:
The Democratic party does not want to save us.
A more apt metaphor is good gangster/bad gangster. "Look, my partner is a psycho. If I let him have his way, he'll not only burn down your store, but kill you and your whole family. I think that's horrible, and I don't want him to do that, but if you don't give me the protection money, I have no way to stop him." The Democratic party is the good gangster, the Republic party is the bad gangster, and at the end of the day they're in the bar splitting the take.
A more radical interpretation of the comic, which I cannot believe the author intended, is that the progressives are our hero, and the inane passengers the centrist Democrats. But we can't expect Democrats to have that level of sophistication.