Lake Mead, some 35 miles east of the strip is the largest man-made body of water in North America. The lake is some 125 miles long and varies in width. It has more shoreline than California. It is at some points hundreds of feet deep.  In 1983 the lake was so full it overflowed the emergency spillway gates, but for the past several years it has been dropping due to drought conditions in the states that make up the Colorado River Basin.   The white “bathtub ring” around the shoreline indicates the high water mark from back then — that’s a drop of more than 150 ft at times.
The lake hosts fishing, water skiing, boating of all kinds, and swimming.
Fishing – The sport fish in the lake include rainbow trout, channel catfish, large-mouth bass, striped bass, green sunfish, and bluegill sunfish.  The entertainment fish include carp that grow to about 24 inches long and congregate around the marinas begging for popcorn. There are charter fishing trips as well as boat rental at the marina in Boulder City.
Boating – To get to the marina, take US 95 south through Boulder City and turn left downhill at the visitor center.  The marina is outside of the Lake Mead fee area.  Speedboats, fishing boats, pontoon boats, jet skis and kayaks are available.   For houseboats you’ll have to head north about 30 miles to Callville Bay.  There are beaches and coves all along the lake where you can pull up and picnic or camp.  There are also other marinas and resorts around the lake, with lodging and restaurants.  Sailing is also popular on the lake.
Swimming, camping – Boulder Beach has a swimming area and a campground.  The campground is quite cozy, nestled under palm trees and other vegetation.  The swimming beach is rocky (take your beach shoes) and the water is generally warm only on the surface, and cool a foot or so beneath.
Scuba – The old dive park, which used to be by the old marina site, is now high and dry.  People do go scuba diving, but the lake is generally murky.  You might come across a sunken boat, but more often you’ll just see junk or sunken trash on the bottom.  The Park Service sponsors cleanup dives a couple times a year, which is a good excuse to go diving.  There’s a dive shop in Boulder City where you can rent equipment.
Caution!  Be aware that temperatures at the lake can get much hotter than Las Vegas.  I was at one of the marinas on a cleanup dive one summer when the Park Service called it off and sent us home because the temperature had gotten up to 122° (that’s 50° C).  My AC blew out on the way and we were dousing ourselves with bottled water trying not to pass out from the heat, then my truck stalled just as we arrived back in Boulder City.  Also the wind is much stronger than in town, especially in the canyons.  It seems someone dies at the lake every year.  Make sure every person has a life jacket if you’re on the lake, and be aware that driving drunk is just as illegal in a boat as on the road.