Current lenses 18-55mm, 75-250mm are my main shooting for this. I've got a 24mm prime and Macro 35mm. I am looking at the 18-135mm as well. But thought the fish eye or 10-18mm would be good for perspective. I can see how the fish eye might not give the best but wanted to throw it out there for advice.
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.
Bear in mind that ultrawides squish verticals, so that massive impressive waterfall in front of you is going to look smaller and have less impact at 10mm than it would at longer focal lengths.