  We revisit the classical but as yet unresolved problem of predicting the
breaking onset of 2D and 3D irrotational gravity water waves. This study
focuses on domains with flat bottom topography and conditions ranging from deep
to intermediate depth (depth to wavelength ratio from 1 to 0.2). Our
calculations based on a fully nonlinear boundary element model investigated
geometric, kinematic and energetic differences between maximally recurrent and
marginally breaking waves in focusing wave groups. Maximally steep non-breaking
(maximally recurrent) waves are clearly separated from marginally breaking
waves by their normalised energy fluxes localized near the crest region. On the
surface, this reduces to the local ratio of the energy flux velocity (here the
fluid velocity) to the crest point velocity for the tallest wave in the
evolving group. This provides a robust threshold parameter for breaking onset
for 2D and 3D wave packets propagating in uniform water depths from deep to
intermediate. Warning of imminent breaking onset was found to be detected up to
a fifth of a carrier wave period prior to a breaking event.
