Oh well, I'll accept the challenge. I'll probably be sorry.
A) Belief in God is not enough to stop people from murdering, on the contrary, it's an excuse that's been used rather often to murder people. The bible is full of God's chosen wiping out pagan nations. After all, we currently have a very devout president, but that hasn't stopped him starting a war.
B) You don't know what the man in the boat would have done if he had been atheist. You may be right that people who don't believe in God are more inclined to utilitarian arguments in trying to solve difficult ethical problems. But it's not logically necessary, and I have seen altruistic atheists and very calculating, self-centred believers.