Witnesses have described it as a wall of water, which washed cars and caravans into the sea and caused several buildings to collapse.
Dozens of villagers and tourists were left stranded by the surging waters and had to be airlifted to safety.
And the Met Office's Philip Avery explained why the floods took everyone by surprise.
"We didn't say Boscastle - watch out," he agreed, even though they had forecast heavy rain and possible localised flooding in the area.
The topography around the village makes Boscastle particularly vulnerable - because it lies in a valley at the confluence of three rivers.
In addition, the ground was already wet because of previous rainfall - and more rain is expected in the area again today.
The floods come exactly 52 years to the day after floods swept through Lynmouth, in Devon, killing 12 people.
BBC Spotlight Cornwall reporter David George said: "There is no electricity or power in the area. A number of properties have been completely washed away and, at one point, a 10-foot wave came down one street. "
"Huge trees lying in the street are being washed into the river and three shops are completely gone, either under water or washed away. "
In places, so much water flowed down streets that roads came up and were broken. People abandoned cars on both sides of the road.
The village is one of Cornwall's most picturesque and historic tourist spots, but its position deep in a steep valley may have left it vulnerable to the elements.
Boscastle stands in a deep coomb where two valleys meet, formed by the rivers Valency and Jordan. A third river, Paradise, also flows through the village.
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