I have done the Lake Mead cruise three times. It really is a super way to send a relaxing hour and a half. The cruise gives you a view behind Hoover Dam which is a view you'll never see unless you do other water activities at the lake.
The narration during the cruise is very informative. It explains the geology of the Boulder Basin on which this section of Lake Mead is formed. The narration goes on the explain the building of the dam, and now there's a separate narration to describe the Hoover Dam Bypass being built south of the dam.
I have done this cruise twice in the summer. The paddle wheel boat is air conditioned, but when it's hot, it's not conducive to going out on deck to get some great photos. More moderate months would be best. If you are visiting in the summer, don't be afraid of going because of the heat. You;'ll enjoy the trip in the air conditioning inside.