The move to block ports appeared aimed at preventing the rebels, known as Houthis, from rearming, and comes after the coalition achieved full control of the skies and bombed a number of rebel-held airports.
The rebels are supported by Iran, but both Iran and the Houthis deny Tehran has armed them.
Saudi Brig Gen Ahmed Asiri, the coalition spokesman, told reporters in Riyadh that naval forces are blocking the movement of ships to prevent weapons and fighters from entering or leaving Yemen. He said they had not yet intercepted anything.
Hadi, who was a close US ally against a powerful local al-Qaida affiliate, fled the country last week, but remains Yemen's internationally recognised leader.
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The US has provided support to the Saudi-led coalition striking Yemen but is not carrying out direct military action. The conflict marks a major escalation in the regional struggle for influence between Saudi Arabia and Iran, which also back rival sides in Syria's civil war.
Asiri said much of the airstrikes on Sunday and Monday focused on slowing the Houthi advance on Aden. He said the Houthis tried to fire ballistic missiles today but that they malfunctioned. Warplanes then struck the force that had tried to launch them, he said.
The Houthis' TV network said the coalition bombed a displaced persons camp in the northern rebel stronghold of Saada, killing 40 people, including women and children.
However, witnesses told The Associated Press that the camp used to house displaced people from an earlier conflict that ended five years ago - is now occupied by Houthi forces and that most of those killed were fighters.
The Houthis and security forces loyal to former President Ali Abdullah Saleh had earlier launched a fresh offensive against Aden, shelling it and battling local militias, but were pushed back by at least two airstrikes, security officials said. Saleh stepped down following a 2011 Arab Spring uprising, but has maintained wide influence through loyalists in the security forces.