Display omitted •Laser ablation of Si was performed both in air and water environments at different fleunces and variable pulsewidths.•Only shallow spallation occurred in air for 10-ps laser pulses, comparing to spallation and phase explosion for shorter pulses.•Shallow laser ablation occurred in water at supercritical peak pulse powers due to laser beam self-focusing and filamentation.•Intense, phase explosion-like ablation takes place during wet ablation by 10-ps laser pulses at sub-critical peak pulse powers. Comparative single-shot pulse study of ablation topographies and depths on Si surfaces excited by 1030-nm sub- (0.3 ps) and pico-picosecond (10 ps) laser pulses at different laser fluences in air and water environments was performed. Fast energy transport via 3D-diffusion of dense electron-hole plasma was revealed at picosecond timescale to extend transversely the ablation craters over the focal spot in air for laser pulse widths shorter, than electron–phonon thermalization time about 3 ps. In the given fluence range fluence-independent shallow ablation was observed in air for 10-ps pulses. Then, rather shallow laser ablation occurred in water at supercritical peak pulse powers, apparently, due to laser beam deterioration by self-focusing and filamentation. Finally, very intense, fluence-dependent material removal took place during wet ablation by 10-ps laser pulses at the sub-critical peak pulse powers. Electron-hole plasma and related energy transport on Si surface, 1030-nm laser pulse filamentation in water, pulsewidth and ambient medium effects were revealed during the surface ablation studies.