Prosecutors are recommending more than two years in prison when the 33-year-old is sentenced Jan. 24 in Seattle.
McDonough was able to break away from the dock Dec. 1 but didn't know how to operate the vessel, which was spotted drifting in Elliott Bay.
In charging papers, prosecutors say McDonough took the $8 million, 480-ton boat "on a whim and as a birthday present to himself." They say his crimes were not only a public spectacle but dangerous and reckless.
According to court documents, he told police he jumped a fence, entered the boat's pilot house and managed to turn on engines using keys left in the ignitions. Prosecutors say he ripped the boat from the dock, causing thousands of dollars in damage, and motored into busy Elliott Bay without lights.
McDonough told police his only boating experience came from piloting a Sea-Doo personal watercraft.
McDonough was arrested by a SWAT team that boarded the boat several hours later. He also told police he was a pirate and planned to flee the country to Victoria, British Columbia.