Do some boats have a cabin but no sole or floors?

Walking on the inside of a wooden boat is what you are speaking of I guess, and stomping around on the ply bottom of some vessels even in bare feet is not wise, but steel and other materials are different, even offshore.
Back in the 70s several Sausalito CA builders made some incredibly minimum and crude ferro cement versions of various boats out of Chapelle. Two were 40' cat-chinese lug pinky schooers and you definitely walked on the hull in those, as interiors were just a plank down the middle, some half FC bulkheads, and a shelf all the way around. These both cruised many offshore miles, and one was lost due to inadequate ground tackle in Mexico.

 

That is the cutest thing I have ever seen float.

 

If hes going through all the effort to build it like a surfboard than he really should look into the "kings cruiser" desighns, with flat(ish) bottoms, too much sail and a love of speed.