I do not think either the T7i nor the 18-135 is weather sealed, so bring a separate waterproof P&S camera if you want to shoot from the boat, at the bottom of the falls.
IMHO, a fisheye is for effect, not perspective.
Look at a native shot from a fisheye and study it, to determine if it is really what you want, before you drop $$$ on one.
A series of shots stitched together would give you a better (undistorted) panorama image.