This paper is concerned with singularities in inviscid two-dimensional finite-amplitude water waves and inviscid Rayleigh-Taylor instability. For the deep water gravity waves of permanent form, through a combination of analytical and numerical methods, we present results describing the precise form, number and location of singularities in the unphysical domain as the wave height is increased. We then show how the information on the singularity can be used to calculate water waves numerically in a relatively efficient fashion. We also show that for two-dimensional water waves in a finite depth channel, the nearest singularity in the unphysical region has the form as for deep water waves. However, associated with such a singularity, there is a series of image singularities at increasing distances from the physical plane with possibly different behaviour. Further, for the Rayleigh-Taylor problem of motion of fluid over vacuum, and for the unsteady water wave problem, we derive integro-differential equations valid in the unphysical region and show how these equations can give information on the nature of singularities for arbitrary initial conditions. We give indications to suggest that a one-half point singularity on its approach to the physical domain corresponds to a spike observed in Rayleigh-Taylor experiment.