I imagine that it would appear either as a large fish symbol, which on some sounders (don't no what sort you have) could be a large school of small fish or just a large fish.
I have seen many strange things appear on sounders and a lot of things not appear on sounders. (I used sounders a lot for navagation) but i think it would be very difficult to pick unless you could pyhiscally see the shark. Large arc to medium arc for all you know i could be a dolphin or a marlin all in all just buy a shark shield for xmas.
I had a new radar i was using once and we had these radar blips coming at us from all angles, took a few minutes to realise we had tuned it in to far and were picking up seagulls.