Cuban state news media say 10 people died across the island as it was being battered by Hurricane Irma.
Most of them died in Havana, where chest-deep seawater pushed several blocks into densely populated neighborhoods.
The state media say several of the deaths occurred in partial building collapses. Much of Cuba's housing stock is deteriorating.
At least 24 people died in other parts of the Caribbean as the hurricane blew through.
Large parts of Havana remained underwater on Monday after waves of up to 36 feet pounded the Malecon, its famous seafront boulevard.
The storm hit Cuba late on Friday with sustained winds of than 157 miles per hour and spent almost 24 hours ripping up the island’s northern coastline for some 200 miles.
Irma was the first Category 5 storm to make landfall in Cuba since 1932.
State media said on Monday Irma had seriously damaged Cuba’s agriculture and tourism industry at a difficult moment, in the midst of sputtering economic reforms and aid from its key ally, Venezuela, hurt by a long-running political conflict.
“Given the immensity of its size, practically no region escaped its impact,” Castro, 86, said in a statement published in state-run media, urging Cubans to unite to rebuild the country.
“The task we have before us is immense but, with a people like ours, we will win the most important battle: the recovery.”
Damage wasn't limited to Havana. More than 100 houses in a small town on Cuba's coastline were destroyed in Matanzas Province when Irma swept through the area, leaving hundreds of people homeless.
In every neighborhood, residents talk warily about the buildings that are one hurricane away from total collapse.
On Galeano Street in Central Havana, a fourth-floor balcony dropped onto a bus carrying Maria del Carmen Arregoitia Cardona and Yolendis Castillo Mart
In photos: Destruction and flooding in Cuba from the passage of Irma
Critics of Cuba's one-party communist-run state said the government needed to loosen economic restrictions to help rebuild. "The Cuban authorities must allow all Cubans to work freely in reconstruction to solve individual needs of the most vulnerable and to get Cuba out of the economic and social crisis created by the inefficient iron grip of the one party government," wrote independent journalist, Miriam Leiva on Facebook.