The Sea Queen (or Queen of the Sea) is a capricious deity. She may or may not be the same deity as Libertina. After Vorbis orders the killing of a porpoise in Small Gods, in order to demonstrate to the crew that they have strayed from the true Omnian path and that their belief is baseless superstition worthy of the attentions of the Quisition, she gets somewhat testy.
For, as any good sailor knows, porpoises are Her beloved children and to slay one brings about divine retribution. A passenger like Vorbis would not know this.
When the expected retribution happens, Om in his tortoise-form bargains with her to spare him and Brutha. She grudgingly concedes that a fellow-god,however insignificant, has the right to save his believer, and spares their lives when the ship sinks.
It is possible that she ensures Vorbis is the only other survivor of the shipwreck either out of sheer spite or an intuitive understanding of Coleridge: A thousand thousand slimy things lived on, and so did I