Either hopping over to the other deck once the other ship is immobilized and pulled against your own, or a boarding plank laid down across the gap.

Instead of being limited to whatever boarding plank can be hoisted across by a gang of burly marines, the corvus made it easier by being a fixed installation - very sturdy, very heavy. The siding helps too. Since it was dropped into the enemy deck, the 'beak' would lodge itself in the wood, minimizing how much the corvus shakes or shifts further.

If you're a fellow who can't walk a straight line on choppy seas even if your life literally depends on it, the corvus is your lifeline.