What does a power boat do when approaching a large vessel?

Stay out of its way! By "large vessel" you mean something like a cargo ship? If you are in a small craft and the other guy is in a ship, assume, unless he gives a signal to acknowledge he sees you (like blowing his horn, flashing lights, etc) that he doesn't see you. If he can't see you and you get in his way, he'll probably squash you flat.