Ocean-sounding breath comes from narrowing the back of the throat so the breath makes a soft sound on its way in and out. People compare it to the sound of waves, or to wind moving down a hallway. Breathe through the nose the whole time, and keep the sound soft enough that someone across the room would miss it.
Ujjayi means victorious, which sounds grand for something you can practice sitting on the couch. Classical texts like the Hatha Yoga Pradipika list it among the key prāṇāyāma practices. The sound gives you a place to focus your attention. When the mind wanders off, the breath is still audible, and you find your way back.
Forcing the sound tightens the throat and defeats the point. In Kripalu Yoga we let the breath lead, so start with an easy breath and add a small narrowing until you can hear it. If your jaw or neck starts to grip, back off. The sound fills in on its own once the breath relaxes.