He's saying that economists who dismiss the effects of recessions and depressions by stating that eventually the economy will be buoyant again are being lazy and useless. By the time the economy has recovered (the storm has passed) and the waters are flat again the ship may have capsized and lives may have been lost.
It's of little or no use to people whose lives and businesses are ruined by economic downturns to simply state that eventually the economy will recover.
It's equally useless for governments to be advised the same because by the time the economy has recovered, if they accept such laissez-faire advice, more damage will have occurred than if they had taken steps to reduce the effects of the downturn and they may, of course, find themselves voted out of office.