If water curves 8" over a mile it could be demonstrated to curve in a fish aquarium. So instead of trying to do a ridiculous 1 mile fishing line across a lake - just show us the miniscule curve in the aquarium

Yeah good luck trying to differentiate a millimetre (less) of curvature from surface tension and menisca effects.

Besides you need to have a bit of showmanship including preferably a couple of bikini babes.