It mostly appears at level 17, and yes, you can see one without having the Silph Scope because it has nothing to do with Marowak. It is a bit rare though.
You need a Silph Scope to see all Pokemon in the Pokemon Tower. Bizzarely, this includes Cubone, though I can't think why, since it isn't a ghost. I've heard its cry before when a ghost appears, though.
No. Before you get the Silph Scope, even Cubones will disguise themselves as Ghosts. I've heard Cubone's cry when a particular Ghost appeared. You can only catch them after you get the Silph Scope.
Cubone/Marowak can also be found on Seven Island and in Victory Road.