This might add to the banter, but ever since I was a child, a "bluewater" vessel was a Ship, not a Boat. I was told that "boats go on ships."
The requirements I've always been taught was there are "suitable" offshore ships and "optimized" ones. It has been well proven that a good boat, properly handled, can endure terrible conditions offshore.
Further, a strong hull, strong hatches with dogs, and ports that are secure and tough (and not too big that you can't pump out the boat if you loose one in a storm) were points. Displacement weighs on the crew and master as a rolling and pitching ship is flat exhausting. A long, deep full length keel is also something I have seen a good property. the final things I would say make an offshore boat, besides design and crew, is the safety equipment and redundant systems.
I took a 28' with 9' of beam offshore this winter to Cayman from Houston, and then back to Cancun and back north along the coast to Houston. This included almost 6 days at drouge and heave-to in rain storms and high winds from that system that flooded the central USA.
Almost anything, In my opinion could make it offshore, but there are "optimized" designs that do better than others.
I reject your reality and substitute my own.
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