"It's the first time I have seen the sea like this, It's huge," he told the AFP news agency as the swell battered the island in the Celtic Sea and power was cut.
One of the island's ferry terminals was destroyed when a tonne of sand that was supposed to be protecting it was hurled onto it, Kerloc'h said.
Waves smashed the bay windows of a thalassotherapy centre in the Breton town of Douarnenez, causing serious damage, and the clinic at Roscoff also lost nine bay windows, although it continued to function.
Homes were flooded and residents evacuated elsewhere on the Brittany coast and waves averaging eight metres hit the Mont-Saint-Michel.
Moving south, there were eight-metre waves in the Vendée, at least 50 metres of sand bank collapsed into the sea on the Cap Ferret peninsula and homes were flooded and residents evacuated in the Landes, where waves as high as seven to eight metres were predicted for Sunday morning.
Weather forecasters maintained an orange alert on the Atlantic coast on Sunday morning and warned that rivers could burst their banks in Finistère, Charente-Maritime, Gironde, Landes and Pyrénées-Atlantiques.
The storms were expected to move north on Sunday with rain forecast in Paris overnight.