Why do u need to know the density of water to determine if something will sink or float?

If something is more dense than water, like Freon or a rock, it will sink.
If something is less dense, like hexane or oil, it will float.

Basically you need to know the density of water when you design marine vehicle. You don't want to build 100 boats just so 1 of them float.

For something to float, it must displace it's weight in water. To determine the weight of water you need to know the volume of water and the density of water. If you have a 1 cubic foot wooden block that weighs 4 lbs. And you know that the density of water is 8.33 lbs/cubic foot. You know that the block will float. It only needs to displace about 1/2 cubic foot of water to displace its weight of 4 lbs. So 1.2 cubic foot of water will be sticking above the water level.