Surface to solid

Hi. I'm using the Autodesk AutoCAD 2021. I've made an enclosed 3d shape out of 3d surfaces. Its a half section of a boat hull. I want to convert this into one solid object. (Like a lump of clay) Solid all the way through. I thought I could use the SURFSCUPT COMMAND but when I do it says "Solid creation failed, no watertight volume detected."
As far as I can see there are no imperfections. I've attached the dwg.
Can anyone advise?
Thanks