Melted polar caps comes to mind.
So do comets, which can contain a lot of water (ice). You might have one or many comets get disturbed out of stable orbit and crash into your planet.
You might also be more gentle about it, and postulate a big ring of ice particles (such as Saturn's rings) that your planet passes through. Imagine the Asteroid Belt, but smaller and wetter (icier). Again, you'd need an astronomical event to bring your rocky planet into proximity with such a mass of ice particles.
Then there's always artificial hydration. Maybe fish aliens are trying to terraform the planet. Let the alien engineers haul in some water!