The Typical Powerboat is Not Seaworthy
The typical production powerboat is far from seaworthy in even slightly adverse conditions. Above is a Cooper Prowler 42′ which is typical of boats intended to sell at boatshows because of impressive interior volume. These boats are huge inside for their length by being high off the water and beamy, both detrimental to seaworthiness.
Note a great deal of boat above waterline and very little below….This boat is not designed for heavy weather! In my opinion this boat is a textbook case of unseaworthiness. Note the flat and shallow bottom, blunt bow, tiny rudders and props, and no keel at all. Also note the huge windows well forward (where they are vulnerable to waves coming over the bow), low freeboard at the bow, and engine room vents below the sheer in the hull side (easily flooded).
As long as you can keep the speed up (at least 10-12 knots) this boat will be reasonably controllable and have adequate dynamic stability. In a big sea where you have to slow down she will make everybody sick with her fast rolling, plus wear you out trying to keep her pointed into the waves. The combination of tiny rudders, no keel (for directional stability), and very high windage will keep her out of control. Loose power and you are in real trouble.
She will probably be fine for inland cruising, but I would not recommend this type for any open water passages.
Below is what I would consider a reasonably capable semi-displacement boat. The Nelson 42′. Note the differences from the Prowler. Lower height above water, higher freeboard at the bow, finer and deeper bow, less beam and more depth to the hull, no flying bridge on top of the pilothouse, huge keel, deeper draft, engine vents in the house side well above the sheer, bigger rudders, and smaller windows set well aft of the bow……
These are just some factors that affect seaworthiness. Besides design the construction, crew preparation/skill, and maintenance of the boat and her systems are also critical in how seaworthy she might be.