The trick to making a water-tight seal has less to do with the actual physical spacing between the objects and more to do with the particular chemistry of the surface. Surfaces that water likes to stick to and spread out are called hydrophilic. Some examples of these types of surfaces include very clean glass or metal. Surfaces that water avoids and will bead up on are called hydrophobic. Many polymer surfaces, including Teflon, are hydrophobic. For hydrophilic surfaces, water loves to spread itself out to maximize its interaction with the surface. In this case, it can form a layer that is only one molecular diameter thick (approximately 3 A). If the gap spacing is more than this, then water will have no trouble crossing the seal. However, if the gap consists of hydrophobic surfaces, then water will behave differently. Then, water will desperately try to avoid increasing its contact area with the surface and will not cross the seal even if the spacing is large. In order to figure out the width for a particular application, one must compare the force acting to push the water through the gap to the force originating from the hydrophobic surfaces to keep the water from wetting them. As long as the hydrophobic force is greater than the force trying to push the water through the gap, the seal will remain water-tight. In certain circumstances, this gap is very large compared to the size of a water molecule and can easily be in the millimeter range.

The ability to maintain an air-tight seal is dependent on the relative pressures across the seal. If there is any gap that is sufficiently large enough to allow air molecules to pass through, then the differences in pressure across the seal will push the air from the higher pressure into the lower pressure until the pressures are equal. In order to make sure that air molecules can not cross the seal, the physical gap spacing must be less than approximately 3 Angstroms, which is the diameter of an oxygen molecule.

Yes, it is possible to get two objects so close together that they are either "air" or "water" tight. This clearance depends essentially on the shape and size of the molecules that you are tying to block (namely H₂O for water and N₂ and O₂ for air). The diameter of a water molecule is about .3nm which is about 3 angstroms, which is about 0.00000003 cm! so in order to ensure that no water got in you would need an incredibly small clearance. For air this distance is even smaller, about .25nm or 0.000000025cm. For this reason it is incredibly hard to make anything which is "air tight", and it is an active area of academic research to develop better vacuums since, because you cannot make anything "airtight" ;the best you can do is such the air that does get in out.