But on Saturday, the men in blue were no less a part of the story at the party's fifth-annual installment. Spurred by multiple fights and drunken rowdiness last year, authorities stepped up the police presence considerably, both along the Dobbins Island beach and in the river.
The Coast Guard for the first time mandated a 40-foot-wide float-free channel leading to the beach in case of emergency, and paramedics were stationed on a county fireboat and in an ambulance on shore.
Last year, an estimated 800 hundred pleasure boats effectively created a wall between Dobbins Island and open water. When a young man passed out on the beach, several people had to carry him 75 yards through the water to reach an emergency services boat, according to two men present.
Without the extra law enforcement provided by the Anne Arundel County police and Maryland Natural Resources Police, Julie Peyton of Arnold would not have returned to the island with her 13-year-old daughter, Emma, for the daylong party.
"We weren't going to come because last year was so bad," said Peyton, joined by best friend Peggy Tennant and Tennant's children, Zachary, 13, and Caitlyn, 10. Last year, Peyton said, the party was "out of control. There were too many fights."
The beefed-up police presence seemed to have a positive effect, police said, without dampening the fun. People frolicked in the warm water and drank cold beer amid a flotilla of several hundred boats that ranged from little runabouts to cabin cruisers.
"We've had a very well-behaved crowd," Sgt. Art Windemuth of the Natural Resources Police said around midday. "No incidents. People are out enjoying themselves, respecting each other, responsibly consuming alcoholic beverages at this point."
Event organizers were not available to comment, because none could be found. Bumper Bash has a Facebook page, but no contact names are listed. An email sent Friday afternoon went unanswered.
The party's amorphous nature is among the concerns of David Clickner, who owns the unpopulated Dobbins Island. At least, he owns everything out to the vegetation line, according to a judge's ruling. The beach itself was deemed public property, and that's as far inland as police let partiers go.
Clickner says he has no objection to the idea of Bumper Bash. But he says it got more unruly after that judicial ruling last year, with ever more boats and people. "The only problem I have is it has gotten out of control — fights, people getting hurt," Clickner said.
And while he agrees with the need for extra police, he thinks the county should treat it as a major event. For instance, because no one required portable toilets on the island, he said, many people probably used the river as a bathroom.
Half a mile from Dobbins Island, several residents of the North Shore community met Saturday to make sure no one bound for the party parked on their private property and that no one from the party tried drunkenly to come ashore — problems they say they've had with past Bumper Bashes.
"It's a fun event, I'm sure," said Sarah Ann Parson, one of the neighbors assembled at the North Shore beachfront pavilion. "They just need to have respect for the island, and the property around and each other."
On Dobbins Island beach, eight uniformed county police officers kept a watchful eye over the party, while colleagues patrolled by boat. Some in the crowd could have done without their presence.
"Everybody's a little afraid to party," said Matt Groseclose of Pasadena.
But he seemed happy enough. He was celebrating a milestone birthday. "Ain't nothing better than this — a Bumper Bash 21st birthday," he said.
Holding a Bud Light in one hand and a cigarette in another, he stood in thigh-deep water playing beer pong on a table set up in the water. All around him people moved through the water or floated by on rafts or inner tubes.
Groseclose's friend Kelly Hickman, 20, said she understood why the police turned out in force. Around 7 p.m. last year, "all of my friends" got into a fight. "Because they were drunk. They all went to jail. They were bloody."
Brian Rent, 36, was happy to see the police. "Everybody's out here to have a good time," he said. "As long as everything stays safe, we're good."
Last year Rent and some friends set up a shade shelter on the beach. A young man they did not know stumbled underneath, vomited and passed out. Because the boats were so tightly packed, several members of his group had to carry the man through the water.
On Saturday, that would not have been an issue as boats clustered on either side of the 40-foot channel.
At midday, with the sun beating down, the biggest issue for many was simply drinking enough water. Peggy Tennant said she had worked out a strategy: "We're hydrating ourselves between beers — drink a beer, drink a water, drink a beer, drink a water."