There are more than a dozen marinas around the lake but the closest and most convenient are the following:
Las Vegas Boat Harbor and Lake Mead Marina
website
702-434-4405
This family owned company has been around since 1957 and they offer just about every type of water craft rental you can imagine from jet skis to party boats. They can also help you with fishing licenses, gear, and directions.
Callville Bay Marina
website
800-255-5561
This full-service marina specializes in house boat rentals but they also offer jet skis, power boats, and more.
Lake Mead Cruises
website
702-293-6180
Instead of renting a boat, why not let someone else drive? This excursion company does paddlewheel boat cruises on Lake Mead that include sightseeing, brunch, and dinner cruises.