I have come to believe that all essays walk in rivers. Essays ask the philosophical question that flows through time -- How shall I live my life? --Riverwalking   Mid-June, on a thinly populated island in southeast Alaska: Pop! Pop-pop-pop! Seaweed polyps burst under my rubber boots -- boots that Kathleen Dean Moore and her husband, Frank, have lent me so I can explore the shoreline with them on this low-tide morning. Still, I slip a little with every step. Same for my wife, SueEllen, who's walking nearby with Kathy, their heads down, trolling for whatever wonders the icy water has left behind. Everything's slimy, shiny, newly exposed. Steaming, almost. It could be that morning, long ago, when salty life first hauled itself onto land. Sponges cling to dripping rocks like spatters of luminous-orange paint. Purple-black mussels cluster by the thousands. In the space of 10 feet, I see