This paper presents a series of ideas to make a step forward in the assessment of flooded ship survivability on a time-dependent basis. An extension of the SEM (Static Equivalent Method) to a Quasi-Static Equivalent method (QSEM) is proposed to take into account the time variable and to address the case of multiple compartments. A capsize index based, conceptually, on static calculations is defined as a function of time and sea state. Capsize is considered in terms of both a first-excursion problem concerning the wave amplitude, and as the persistence of high-runs of waves above a critical level. An example of the proposed methodology is presented for the analysis of the flooding a two decks. Using the same hydraulic model, comparisons are carried out between the presented approximate analytical methodology and direct numerical simulations of the flooding process to assess the effect of downflooding means. A discussion is given on how the time-based approach to ship survivability links with the evacuation process, in order to obtain the probability density function of the number of evacuated passengers.