Is Saint Brendan the patron saint of the US Navy?

Not particularly, St. Brendan is the declared patron of boatmen; mariners; sailors; travelers; navigators;whales; the diocese of Clonfert; and the diocese of Kerry. While perhaps not an official patron of the US Navy, he is in everything but name. There are religious medals struck invoking St. Michael as a protector of Navy men as St. Michael is the patron of officers in battle. You can view such a medal via the link below.