Officials say that number isn't unusual. Thursday's incidents do underscore the potential dangers of boating and swimming in Southern Nevada's lakes, particularly during the busiest time of the year.

More than 200,000 people are expected to visit the park for the holiday. Lake Mead spokeswoman Roxanne Dey estimated there will be about 5,000 boats on both lakes.

Officials at the recreation area handled a total of 21 fatalities last year, including several car crashes, a suicide, and other incidents unrelated to water activities.

Dey said the number is low considering 8 million people visit the two lakes annually.

"Do I wish the numbers were lower? Absolutely," Dey said. "A vast majority of people have a safe and fun time on the water. We don't like to dwell on the small percentage that don't because we don't want to see (deaths) happen."

Statistics for fatalities through early July have held steady in past years. Sixteen people died in the park last year and 18 died in 2006 during the same period.

Adding to this year's toll was the death of Elena Vlad, 62, of Constana, Romania. She was boating on Lake Mead with her son at about 4:30 a.m. Thursday when their vessel struck land and ejected them both.

A few hours later, 49-year-old Calvin A. Cummings of Bullhead City, Ariz., suffered a heart attack and died while boating with his family on Lake Mohave.

Nevada law requires anyone under the age of 13 to wear a life jacket while boating, but poor swimmers should don them in the event of an emergency, said Martin Olson of the state Department of Wildlife.

Officials said Vlad was wearing a life jacket when she was ejected from the boat, but hit her head on nearby rocks. She was pronounced dead en route to the hospital.

Cummings was standing next to a boat on Lake Mohave when he suffered an apparent heart attack and fell into the water. His friends pulled him from the water and transported him to meet emergency officials near his home in Bullhead City, Ariz.

He was pronounced dead at the Western Arizona Regional Medical Center.

Rangers caution boaters and visitors to be safe in the parks and ensure vessels are in good mechanical order before hitting the water. Olson said a full staff of wardens will be on hand to assist park rangers in monitoring boat traffic and shoreline activity.

Contact reporter Maggie Lillis at mlillis@reviewjournal.com or 702-383-0279.