But an Iraqi border guard said two of the attackers were arrested while the rest fled to Iranian waters.
The robberies took place about 145km from Basra, an area where seaborne attacks have been quite rare.
The attacks did not formally qualify as piracy under the UN Convention on the Law of the Sea because they took place in Iraqi regional waters rather than the high seas.
The line between politically motivated attacks and crime has become increasingly blurred in Iraq, as various armed groups use robbery to fund their operations.
As the US military prepares to formally halt combat operations on August 31, ahead of a full withdrawal next year, Iraq is seeing about 15 attacks from each day.
Elsewhere in Iraq, three people were killed in two separate attacks in Baghdad, police said on Sunday.
At least 18 people died in a spate of attacks over the weekend, including an Iraqi pharmacist who just returned from studies in the US, officials said on Sunday.
A five-month-long political impasse after a parliamentary election on March 7 failed to produce a new government.