The coalition, which is fighting an Iran-backed insurgency by Houthi rebels, has denied it attacked the boat near the port of Hodeidah in the Red Sea.
But a 185-page report to the UN Security Council has reportedly concluded that they were responsible for the deaths of 42 people. Another 34 people were injured.
"This civilian vessel was almost certainly attacked using a 7.62 mm caliber weapon from an armed utility helicopter," according to the report.
"The Saudi Arabia-led coalition forces are the only parties to the conflict that have the capability to operate armed utility helicopters in the area," the report said.
Two other incidents in March led to the deaths of 11 people in fishing vessels in the Red Sea.
Yemen is in the throes of a civil war that began in 2015.
The conflict pits government troops headed by Yemeni President Abd Rabbuh Mansour Hadi against Houthi rebels, who are backed by army units loyal to former Yemeni President Ali Abdullah Saleh, who was president of pre-unification North Yemen in 1978-1990 before becoming president of the unified country between 1990 and 2012.
Since March 2015 a Saudi-led coalition has been carrying out airstrikes in Yemen.
The coalition, which is armed by the United States and Britain, includes Egypt, the United Arab Emirates, Bahrain, Kuwait, Jordan, Morocco, Senegal and Sudan.
On July 26 a Campaign Against the Arms Trade spokesman, Andrew Smith, told Sputnik British arms were being used for attacks on civilians in the Yemen.
"UK arms have already been used against civilians in Yemen. Right now UK-made fighter jets are dropping UK-made bombs. The bombardment has had a devastating impact on Yemen, killing thousands of civilians and destroying vital infrastructure… At present, the UK, and others, are arming and supporting some of the most brutal and oppressive regimes in the world," Mr. Smith told Sputnik.
The UN report said the attack on the migrant boat violated international humanitarian law.
The Saudi government has not responded to requests for information by the UN in relation to the attack.
"Attempts to divert responsibility in this manner from individual states to the Saudi Arabia-led coalition may contribute to further violations continuing with impunity," said the UN report on the March incident.
Last month dozens of people were killed in the north of Yemen during an airstrike, carried out by the Saudi-led coalition.
"The world is silent about the crimes committed against Yemen by the international coalition and terrorist organizations financed by certain countries," Brigadier General Aziz Rashid told Sputnik at the time.
Peace talks have stalled and the fighting in Yemen has triggered a huge humanitarian crisis, with thousands dying from cholera and starvation.
Migrants fleeing Somalia and Eritrea often cross the Red Sea in an attempt to reach the oil-rich economies of the Arabian peninsula.