A few suggestions... Possibly, the propeller, at least on the nearest plane, should be a bit more visible, although an invisible propeller is much better than a visibly motionless one. Since the planes and the boat are moving at different speeds and in different directions, a bit of blur for the planes would be appropriate. That might involve separate renders. The planes are in formation, but they are a bit too perfect, so some sideways rotation would be beneficial. The smoke from the stack should, perhaps, trail backwards more, since the boat is moving (that depends on how fast the boat is going, although the aircraft would also disturb the plume of smoke with their propellers). You do have a man on the ship, but I'm not sure if the planes have anyone in them. The light on the surface of the sea is pretty strong, while the shadows under the biplanes are also a bit stark, so reducing the strength of the shadows and of the light source may be worth experimenting with.