Police say Dzhokhar Tsarnaev also wrote that he and his brother carried out the attacks for revenge for what he called U.S. attacks against Muslims in Afghanistan and Iraq.
An official showed ABC News a cellphone picture of the phrase "Praise Allah" that was also written in black ink on the boat's side panel. Tsarnaev wrote that the marathon victims were collateral damage, just like Muslims have been during war.
Authorities believe the younger Tsarnaev brother wrote the remarks while hiding in the boat in Watertown, Mass. Tamerlan Tsarnaev had been killed hours earlier during a shootout with police several blocks away.
The Tsarnaev brothers are accused of setting off two bombs near the finish line of the Boston Marathon on April 15, killing three people and injuring more than 260 others.
According to U.S. Attorney General Eric Holder, the Russians have been cooperating with the U.S. in their investigation. U.S. officials are trying to determine whether Tamerlan Tsarnaev was trained by militants during a visit to Dagestan, a Caspian Sea province of Russia that has become the center of a simmering Islamic insurgency.
ABC News and The Associated Press contributed to this report.