Undersuck

Eliot, however, held him up from drowning until one of the boats arrived on the scene, and they were dragged into it, but even then they were in peril from the greatness of the waves, and the undersuck, as they broke and retreated, which well-nigh swamped the boat and its contents, and it was with great difficulty that the men on shore grappled the thwarts of the boat and brought her safely to land, […]