I believe in altruism, but I don't believe that Elon Musk is altruistic. In fact, the way he seems happy to treat his people suggests a far less humane side, but that's neither here nor there. In any case, I don't really care about his motives, just his plans.
He's made some interesting points about using Mars to shield from radiation, but it doesn't matter because they'd be cooked before they ever arrived. Worse, it seems that radiation is going to have an impact on cognition, which is just about the worst thing you can imagine. Dying in your 50's from cancer is tragic, but dying because you stopped thinking clearly before your mission ever truly begins? Wasteful.
The notion of using water as a shielding works at first glance, but then you realize that you'd need a looooot of it, and that you've kicked the problematic can. Now your biggest concern shifts to micro-impacts, which no longer just erode your armor (you'll need armor... expensive as it will be to lift, you'll need it), but open your radiation shielding to vacuum. Presumably you'd have some means to circulate the water through the outer crew quarters, so large parts suddenly freezing and slowly sublimating away strike me as catastrophic.
What we need is a material that is incredibly strong, durable, manages thermal stress, and which can shield against all kinds of radiation (without just cooking the poor bastards with gamma rays secondary to the incident radiation). That material need to be light enough that lifting it into orbit doesn't bankrupt any potential operation, and frankly... no such material exists.
That is just one problem.
Edit: To be clear, I don't think that Elon Musk being something other than altruistic, is for example, malevolent. I think of all the arena in which he could have acted, he chose ones that were potentially beneficial to the species. Not being altruistic doesn't mean that you have to be terrible... most of us aren't altruistic.