When salt makes the water more dense how does it make things float?

Because salinated water has more atoms per mole of the compound, it makes this solution denser than pure water. The heavier a solution is the more force that can be applied to it without breaking its surface. Think of trying to float in air, or to float in water. You are able to float in water because the substance is much denser than air. You will have a much easier time floating in the Dead Sea (the most salty body of water in the world) than floating in a fresh water body of water, which has much less salt in it.