I had good luck with a fake fish decoy and a kingfisher.
Unfortunately we haven't been able to find the cheap, orange
plastic goldfish first marketed as a 'heron scarer' (apparently
the heron was supposed to be scared when it grabbed it
and found it anchored to a rock). The kingfisher sure got
PO-ed about not being able to pull the decoy out of the
pond after six tries.
There are other fake fish out there but they are sold as
decorative items.

k :-)