As far as motorboats go, I would suggest that the key difference between 'displacement' and 'semi-displacement' is the treatment of the aft underwater lines. A true displacement
hull will have a smooth run aft rising towards the waterline, encouraging water
to flow around and past the hull with the minimum of disturbance - in that respect rather like most cruising sailboats.
A semi-displacement hull will invariably have much flatter aft sections that do not rise progressively towards the waterline, but continue almost horizontally until truncated by a fully immersed transom. The effect is twofold: the flat lines aft mitigate against squatting as speed increases and the sudden termination of those lines at the immersed transom encourages the water
to break away from the hull cleanly leaving the characteristic clean wake of a planing hull.
As others have remarked, such designs usually have a fine entry and may or may not have a hard chine. They are usually sea-kindly, not particularly fuel-efficient, and invariably wet in any sort of sea! However, the fact that nearly every modern commercial
Pilot launch has this type of hull speaks volumes for the seaworthiness of this type of hull.