Commentary Inf XXII 19-30

The two similes describe the actions of the barrators in motion and at rest. They only move to relieve their pain for a moment or to duck under the pitch (thus increasing their pain) in order to avoid a hooking by the devils -- a pain still more disturbing, as well as embarrassing.

It was apparently a current belief that dolphins approached ships when they sensed that a storm at sea was brewing. See Chiavacci Leonardi (Chia.1991.1), p. 658, for references in Brunetto Latini (Tresor I.134) and Jacopo Passavanti (Lo Specchio della vera penitenza, Florence, 1725, p. 257).