Why does a metal boat float? I was in class learning about density and stuff. Our teacher told us that things that are denser than water sink in water, and less dense things float. 
Then, our teacher asked us why metal boats float in water, even though they are denser than water. Is it because of the surface tension of water? Some other thing?
Any help would be appreciated.
 A: This is because the whole boat, along with the air in the boat, is lighter than the water it displaces. 
For example, if a small boat will take up 1 cubic meter of water, then it has to be heavier than the weight of 1 cubic meter of water. This is explained in this post by What If here.
For the same reason that bowling balls float (because salt water the size of a bowling ball weighs more), boats float (because the overall weight of a boat is less than the overall weight of salt water the size of a boat.
A: 
It seems like they ought to sink because we're used to seeing things fall. But for the ship to sink it has to push aside some water, which has nowhere to go but up. So it's a question: does the ship 'want' to sink more than the water 'wants' not to rise? It turns out that just depends on whether the ship weighs more or less than the amount of water that would fill the same space. Real ships have lots of air inside, so they weigh less than the same volume of water, so they float.


Here's a cool experiment you could try. Fill your sink with water, then put a bowl in it. Now see how much weight you can put in the bowl before it sinks. You could even pour in water as the weight. If you use water, look at how high the water comes up on the inside and on the outside of the bowl as you add more water. Is it the same or different?
And by the way, isn't it cool that it doesn't matter how deep or shallow the water is? If something floats in 2 feet of water, it will float in 2000 feet of water!
I hope this makes sense.

(Source)
A: Two factors:
Overall density. A boat isn't only metal. It contains copious amounts of plastics and wood, which have lower densities than water. You have to calculate the overall density of the object, not just the metal.
Shape: If boats were made in a sink-friendly shape, they would certainly sink (regardless of wood and all). Engineers design boats to be as much float-friendly and sink-unfriendly as possible. Imagine a boat which has a steep V-shape with a very narrow angle. This boat is far more probable to sink than one which is also a V-shape but has a much wider internal angle.
A: Displacement. Until the Vessel weighs more than the volume of water it is displacing it will remain afloat.  Poke a very large hole in it.. Iceberg, torpedo  or in the case of the Costa Concordia Sight seeing, it will most likely sink. Think of a Hydrogen filled balloon.  It weighs less than the volume of air it is displacing so it rises.. 
Poke a hole in a ship the area affected is no longer displacing water, but the weight of the vessel has not changed. 
A: If you put something in water that has a higher density than water it will sink. If that thing with everything in it has a lower density than water, it will float. What material that thing is made of is irrelevant.
A ordinary boat is mainly filled with air, which is a lot less dense than water, so even if the boat is made of lead, it will float. If you replace the air in the boat with water, it will sink.
At the bottom of seas and oceans lay a lot of wooden shipwrecks. By itself, that wood might float, but because it is part of a ship with a higher density than water, it sunk anyway.
The same thing goes for water itself. High density 'heavy' water sinks, light water floats on top. Fresh water has its highest density at roughly 4 degrees Celsius. That is why the temperature of the deepest part of a lake that is frozen over, always has that temperature. That is how fish survive the winter, in the mud on the bottom. Water has its lowest density in its solid form as ice. That's why ice floats on liquid water, like a boat.
