According to a USA Today timeline:

"CNN is reporting that the Coast Guard has fired on a boat on the Potomac River.
Update at 10:10 a.m. ET: The report of some activity on the river comes only minutes after President Obama concluded 9/11 memorial ceremonies at the Pentagon not far away.
Update at 10:12 a.m. ET: CNN says the Coast Guard fired 10 rounds at the boat.
Update at 10:14 a.m. ET: CNN broadcasts what it says is audio of the Coast Guard apparently speaking to a suspicious boat. The Coast Guard speaker is heard saying; "Slow down or you will be fired on."
Update at 10:27 a.m. ET: The AP quotes the Coast Guard as saying training was being conducted in Potomac River moments before Obama motorcade crossed.
Update at 10:30 a.m. ET: CNN quotes two police sources as saying the incident was a possible Coast Guard training exercise.
Update at 10:32 a.m. ET: Fox News quotes Coast Guard Chief Keith Moore as saying no shots were fired as part of the exercise.
Update at 10:33 a.m. ET: Reuters quotes an FBI spokeswoman as saying the agency was told by the Coast Guard that no shots were fired at a boat in the Potomac.
Update at 10: 41 a.m. ET: NBC's Pete Williams, quoting a "senior Coast Guard official," says the incident was just an exercise to "challenge response times."Williams says the exercise was strictly a "radio exercise" and did not involve any boats or any firing by the Coast Guard. Williams reports that a "citizen" overheard the radio exercise and passed that along to some news organizations, which broadcast reports of possible shot being fired.
Update at 10: 53 a.m. ET: USA TODAY's Kevin Johnson reports that the Coast Guard say that no shots were fired and that the incident involved a "radio exercise." Johnson says the Coast Guard is preparing a statement for release in a few moments, which we will carry."

The AP's version as of 11 a.m.:
"WASHINGTON — The Coast Guard conducted a training exercise in the Potomac River near the Pentagon amid Sept. 11 commemorations Friday, sparking confusion.
Coast Guard Chief Keith Moore said Friday no shots were fired as part of the exercise. Media reports suggested shots had been fired in the river and showed vessels circling in the water, near the bridge where President Barack Obama's motorcade passed as he traveled to a Sept. 11 memorial at the Pentagon.
Coast Guard spokesman John Edwards said references to shots fired were picked up in radio chatter. As part of its exercise Friday, the Coast Guard aired simulated instructions to participants to fire 10 rounds. But Edwards said there were no shots actually fired and there were no suspicious boats.
The Coast Guard conducts this sort of training everyday. The training had nothing to do with the 9/11 anniversary, Edwards said.
"This is routine training for us and we train everyday," Edwards said.
Obama had traveled to the Pentagon to lay a wreath in a ceremony commemorating the eighth anniversary of the terrorist attacks."

Might want to take this particular day off, knuckleheads. This is basically the equivalent of Spain testing its bomb threat response time at train stations on March 11.