Crocodiles and (IIRC) some Whales. The problem is that I cannot see them - I send my ship ahead and I see messages that it has been ambushed (or attacked between turns). Now my anty-fish force consists of 27 merged Triremes, 9 merged Sea Turtles (they turned out to be very slow, so finally I have sent them to autohunt) and I am preparing 27 merged other UU's - Decareme replacement with March, Maneuvring (sp?) I and IIRC some other free promo. Thanks to buildings, small and great wonders, my ships have also some other free promotions (Sea Hunter I, Combat I, all possible Mesopotamism assistances), so this gives them another boost.