To give an idea of distance:
The pictures below were taken with a Fuji T300 Point and Shoot last week. I'm not sure how wide it is, but it was set at it's widest setting, so probably 24 to 35 mm. These shots were taken from the Maid of the Mist. The boat was in the middle of the river. If you were on shore, you would be twice as far away, so I think if any thing, 24-70 may be too short for some shots, but it will certainly be wide enough. I have a Rebel that I was not willing to take on the boat (and I'm glad I did not) but there was at least one person on there with a 60D with no protection.
If you've never been on it, the Maid of the Mist approaches the Horseshoe Falls and goes full throttle until the current keeps it from going any closer. They hold the position for a few minutes. Depending on wind, you will stay fairly dry approaching the falls, but when I was there, it changed very quickly as we approached. It was like standing in a massive rain storm. Just be prepared as the conditions change quick. Whatever you do use, don't forget the lens will get wet and mess up the photos, so be prepared to be able to wipe it off.
The boat rocks around a lot and there will probably be alot of people, so it really isn't the best photo place, as I think you may have stated above. The provided photos were of the American Falls and it was whatever I was able to see on a bright day on the EVF, handheld on a rocking boat with a mist coming now and again and peoples heads in the way. As you probably already know, your best shots will come on land.
You can also get some nice shots from the Cave of the Winds. It depends on the wind, but there are places you can take pictures and stay fairly dry, but you never know when the mist will come. However, you will pass through the Hurricane deck where you will want protection for your gear (good protection). They say the conditions vary, but when I was there it was more of an experience than actually something to see as the was was so intense you had to squint to see anything at all.
Make sure to take photos from the observation deck on the US side. Consider a trip to the Whirlpool State Park for a different perspective. You can hike down to the river, but the guide said it is very tricky and do not go in the water (it is illegal and you will be fined $3000 if you survive - they loose 2 people a month). There is also a cable car on the Canadian side that goes over the whirlpool as well as a jet boat (you won't want to take a camera here!).
As stated, the Canadian side has better views and better attractions. The tower is also very nice. They have a dining floor that rotates once an hour. I went in years ago and tried to time it for sunset.