Commentary Par XXIV 39

Peter's walking on water displayed his faith in Jesus, but also revealed the tenuous nature of that faith when he doubts and begins to sink, causing Jesus to castigate him: 'O you of little faith' (Matthew 14:28-33).  And so here Beatrice is remembering Peter's noble beginning and suppressing reference to the far less impressive conclusion of the biblical narrative.  See C.Par.XXIV.124-126.