The Swamp was Upside Down by Murray Leinster

The leeward side of the island went very gently into the water. From / a boat offshore -- say, a couple of miles out -- the shoreline looked low / and flat and peaceful. There were houses in view, and there were boats / afloat. But they were much smaller than those that had been towing a / twenty-mile-long oil-slick out to sea. These boats did not ply back / and forth. Most of them seemed anchored. On some of them there was / activity. Men went overboard, without splashing, and things came up / from the ocean bottom and were dumped inside their hulls, and then / baskets went back down into the water. At long intervals -- quite long / intervals -- men emerged from underwater and sat on the sides of the / boats and smoked with an effect of leisure.

There was sunshine, and the land was green, and a seeming of / vast tranquillity hung over the whole seascape. But the small / Survey-personnel recreation-boat moved in toward the shore, and the / look of things changed. At a mile, a mass of green that had seemed to / be trees growing down to the water's edge became a thicket of tumbled / trunks and overset branches where a tree-thicket had collapsed. At half / a mile the water was opaque. There were things floating in it -- the / roof of a house; the leaves of an ornamental shrub, with nearby its / roots showing at the surface, washed clean. A child's toy bobbed past / the boat. It looked horribly pathetic. There were the exotic planes / and angles of three wooden steps, floating in the ripples of the great / ocean.