Animating a ship on sea surface

Start by opening an image close up of ship in sea.

Create selection of the ship using polygon lasso tool. Press Ctrl+J to copy paste the selection in a new layer.

Hide the layer. Select background layer. Select clone stamp tool and clone the boat area.

Duplicate the background layer twice. Press Ctrl+T. Right click and select Flip Horizontal and place on either sides.
Select and merge three water layers. Unhide the ship layer.

Duplicate the ship layer twice. Rotate each layer by few pixels clockwise and counterclockwise respectively.
Open animation window. For the first frame hide all three boat layers.
Duplicate the frame. Move the water layer towards right.
Click tween button from animation window. Apply settings as shown.

Select first frame and unhide the boat layer.

Select 2nd frame and move ship layer a pixels downwards.
Select 4th frame. Unhide the second boat layer and hide the first boat layer.
For every other frame unhide the other ship layer to create random motion effect.
Press Ctrl+Alt+Shift+S to save an optimized animated GIF.