Four-Day Planet by H. Beam Piper

There was a man in the turret, waiting to help me. He had a clip of five rounds in the gun, the searchlight on, and the viewscreen tuned to the forward pickup. After checking the gun and loading the chamber, I looked in that, and in the distance, lighted by the boat above and the searchlight of the _Javelin_, I saw a long neck with a little head on the end of it weaving about. We were making straight for it, losing altitude and speed as we went.

Then the neck dipped under the water and a little later reappeared, coming straight for the advancing light. The forward gun went off, shaking the ship with its recoil, and the head ducked under again. There was a spout from the shell behind it.