A dark shadow maneuvers through the waves, and in less than a second of people spotting it, a serpentine head brakes through the water surface and hisses. Lester can hear the beast from bellow the deck, and doesn't doubt that people a mile away can too. Men rush by him in the gun deck, frantic and waiting for orders. 

"I know the creature is already in front of us, but it will do no good to fire until the ship has been steered a bit to the side. I need powder monkeys down here now. We need to move quick!" 

He moves toward a small opening to get a good view of the beast. It's large, probably longer than the ship with a head large enough to swallow five or so men at a time. It's a sickly green and looks ready to strike. Hopefully the swivel cannons can delay an attack until the ship is adjusted for proper cannon fire.