I think it is weird that Sayid suddenly became a good guy when on the sub. Obviously it fulfills our hope that he got a chance to redeem himself, so I've tried to make sense of it. When Jack pushed MIB in the water, maybe it diffused his power. I'm sure some people will argue that he does not have "mind control," however, Claire seemed to gain some sanity as she spent some time away from MIB, so I think there's something to it. I think this detail about the water is important because it may be one way for Jacob or whomever to use it as a weapon against him. Maybe not exactly like the Wicked Witch of the West, though along the same lines.