I have seen "our" heron(s) actually land in the water, just off shore. I think I'd place the electric fence as close as possible to the water, watch for the effects on the heron, and move it accordingly. If you can catch him with his feet in the water, he's going to get zapped pretty good.

"Live like you'll die tomorrow, but manage your grass like you'll live forever."
-S. M. Stirling
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