Question

does something have to be more or less dense to float?

Answer 1

Ususally a less dense material will float. This is because the weight of the less dense material is less than the weight of the liquid it would displace if it were submerged. If you look up Archimedes' Principle you'll ge the idea:

http://en.wikipedia.org/wiki/Archimedes%27_principle

However, ships are usually made of steel so how do they float? Well, they displace water (move it out the way) using the strength of the hull and have a less dense material inside (air) so overall, the weight of the water displaced is equal to the total weight of the ship and it sits in equilibrium on the water.