Polarity of the nitrate ion
Don't overthink it.

The oxygens have a partial negative charge in nitrate from resonance.  The protons in water have a partial positive charge from dipoles.  Positives attract negatives.  

Molecule polarity becomes an issue when discussing neutral molecules interacting with neutral molecules.  Molecular polarity is one thing, and bond polarity is another.  Transformations / arrangements / alignments of molecules such that the dipoles (the things that make a molecule or bond 'polar') align to maximize interaction are certainly important for dissolution, but when a species is a bona fide free ion, water will tend to dissolve it.