(arabiboatworks.com)

All boats rely on simple physics principles to work.  No matter how big, small, fast or slow a boat is, they all rely on these same concepts.

First, they rely on reaction forces.  Water is a heavy substance weighing in at about 62 pounds per cubic foot (give or take).  When people build boats, they calculate something called the Displacement.  The displacement of a boat is how much water it will "push out of the way" when it is floating ("Displacement", Wikipedia).  You can make a boat with as large or small of a volume as you like, but its displacement must be less than what the same volume of water would weigh.  The reason for this is that the amount of force that the boat exerts on the water is the same as the amount of force the water exerts on the boat.  As long as the boat's displacement is less than the water, it will float.

However, if you build a boat and accidentally make it heavy enough so that its displacement is greater than the same volume of water, you will find that your boat has become an anchor! The reason is that once the boats displacement exceeds that of the water, the force of the boat pushing on the water is greater than what the water can push back with and the boat overcomes that force.  This is what makes a boat sink ("Displacement", Khaja, A.).

The bottom line is that what makes a boat float is the action-reaction forces between the water and the boat hull.

(background photo URL: thoughtontracks.com)