Are there water molecules in the oceans which will almost never fall down as rain?
Never is a very, very long time.  Almost never is a vague term.

The age of deep ocean water has been studied with both models and observations, and both indicate a lower limit age in the order of thousands of years, rather than decades.  Here, age is defined as elapsed time since contact with the surface, which would be the only opportunity for evaporation.

On a geological timescale, all ocean water has ample opportunity to evaporate and fall back as rain again and again.  On a human timescale, not so much: for some water in the ocean, it has been well over a thousand years since the last time it was near the surface.

For an example of a recent study into the topic:


  Gebbie and Huybers (2012): The Mean Age of Ocean Waters Inferred from Radiocarbon Observations: Sensitivity to Surface Sources and Accounting for Mixing Histories.  in: Journal of Physical Oceanography.  http://dx.doi.org/10.1175/JPO-D-11-043.1


From the abstract (emphasis mine):


  A number of previous observational studies have found that the waters of the deep Pacific Ocean have an age, or elapsed time since contact with the surface, of 700–1000 yr. Numerical models suggest ages twice as old. Here, the authors present an inverse framework to determine the mean age and its upper and lower bounds given Global Ocean Data Analysis Project (GLODAP) radiocarbon observations, and they show that the potential range of ages increases with the number of constituents or sources that are included in the analysis.
  (...)
  The authors find that the North Pacific at 2500-m depth can be no younger than 1100 yr old, which is older than some previous observational estimates.
  (...)
  Subject to the caveats that inference of the mixing history would benefit from further observations and that radiocarbon cannot rule out the presence of extremely old waters from exotic sources, the deep North Pacific waters are 1200–1500 yr old, which is more in line with existing numerical model results.