Thirty yards out a shark fin, steel-gray, triangular in shape, cut across the swell, then disappeared under a wave. Jimmie and I stood up on the sandbar, our hearts beating, and waited for the fin to resurface. Behind us we could hear the crackle of lightning in the clouds.

"It's probably a sand shark," Jimmie said.

But we both knew that most sand sharks were small, yellowish in hue, and didn't cruise at sunset on the outer shelf. We stared at the water for a long time, then saw a school of baitfish scatter in panic across the surface. The baitfish seemed to sink like silver coins into the depths, then the swell became smooth-surfaced and dark green again, wrinkling slightly when the wind gusted. I could hear Jimmie breathing as though he had labored up a hill.

"You want to swim for it?" I asked.

"They think people are sea turtles. They look up and see a silhouette and see our arms and legs splashing around and think we're turtles," he said.

It wasn't cold, but his skin looked hard and prickled in the wind.

"Let's wait him out," I said.

I saw Jimmie take a deep breath and his mouth form a cone, as though a sliver of dry ice were evaporating on his tongue. Then his face turned gray and his eyes looked into mine.

"What?" I said.

Jimmie pointed southward, at two o'clock from where we stood. A fin, larger than the first one, sliced diagonally across a swell and cut through a cresting wave. Then we saw the shark's back break the surface, a skein of water sliding off skin that was the color of scorched pewter.

There was nothing for it. The sun was setting, like a molten planet descending into its own smoke. In a half hour the tide would be coming in, lifting us off the sandbar, giving us no option except to swim for the beach, our bodies in stark silhouette against the evening sky.