I think the difference lies in the fact that PT boats did around 30-40 knots, which would have them slicing through the waves. Most crewboats working in the oilfields (the ones I were on anyway) do less than 20, which ride the waves up and down. A steel hulled supply boat would hardly feel the seas due to its weight and size. A crew boat, on the other hand, is much lighter (like the PT boat) and you feel the waves much more. Those smaller boats (65 +/- feet), like the ones on that website, are used for short runs to platforms 10-20 miles out and do not weather strong seas.