“…The theory of water waves now is among the most actively developing branches of the hydrodynamic science (see, for instance, [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16], and many references therein). In particular, two-dimensional (2D) flows of an ideal incompressible fluid with a free surface are very attractive for theoretical study, since they possess many essential features in common with real water waves, and simultaneously are relatively simple for analytical and numerical treatment.…”