Are all ionic substances soluble in water?
Absolutely not. A great many ionic species are INSOLUBLE in water.
Ionic compounds are non-molecular. They consist of a rigid array of positive and negative ions, whose attractive electrostatic interaction persists throughout the ionic lattice. A measure of this ionic interaction can be measured (both experimentally and theoretically) by the lattice enthalpy.
In earlier studies of chemistry we learn that all carbonates, phosphates, and oxides are insoluble in water (I think you know the exceptions). Water is a good solvent for ions inasmuch as it can effectively stabilize the individual cations and anions. Nevertheless, there are a great many ionic species that are insoluble in water, and this is attributable to the strength of the ionic interaction: ions with charges greater than
Of course, network covalent solids are also non-molecular, and the individual particles are bound together by strong covalent bonds. You would have a hard time dissolving graphite or silicon dioxide in water.
I am certainly willing to expand on the subject. It depends whether you are in A level or are an undergraduate.