1. Air pressure is a measure of the WEIGHT of air above a particular area. Thus, it depends on the gravitational force of the earth on a certain amount of air. Absolute pressure is a measure of force of molecules hitting an area. It thus has to do with the amount of molecules, speed of the molecules (temperature) and the mass of the molecules (nitrogen vs oxygen etc.)

For instance, if you were to take the exact atmosphere (same amount of air, same temperature, etc..) and put it on the moon, the smaller mass of the moon would exert less gravitational force on the air and thus it would weigh less, and we would experience less air pressure.However, a closed container of air at the same temperature has the same absolute pressure on earth and on the moon.

2. Ships do not sink due to the buoyant force. This concept works on the principle that for a ship to sink, it would need to displace one ship's volume worth of water. To displace this water takes a force equal to the weight of this volume of water. This is the reason very light things (styrofoam, beach balls etc) float and dense things (metal etc.) sink. However, in the case of ships (think about the titanic) we play a trick on water. The volume of water displaced includes the hull of the ship plus all of the air inside of the hull (which adds a lot of volume and not a lot of weight. If there is a crack in the hull letting the water in and pushing the air up and out, then the volume of the ship is reduced to the metal itself and it sinks. in this case you only displace a small amount of water. [sorry, it is a bit difficult to articulate this concept, but think about when a beach ball is inflated or deflated]

3. Flames are hot (of course) and thus heat the air around them. This hot air is less dense than colder air, and thus rises. (So why don't they continue to rise you ask?) As they rise, they are cooled down, to the point where you can't see light, but can still feel heat, and further away, where you can't feel heat at all.