I'm not an expert but I can make an educated guess. They put dimples on golf balls to create turbulence, which gives less drag than laminar flow over the ball.

I'm not an expert but I can make an educated guess. They put dimples on golf balls to create turbulence, which gives less drag than laminar flow over the ball. I assume that is the same thing happening in the sharkskin swimsuit. For the ship hull I have another guess (sorry). In fluids, you can't reduce "friction" by having a "slipperier" surface. The rule is (for laminar flow) the speed of water at the surface, relative to the surface, is 0. That determines the velocity field around the object, which in turn determines the drag. My guess is that when the air is between the ship and the water, the air has 0 velocity at the surface of the ship. The air also has a 0 velocity relative to the water at that boundary. But the air has a velocity gradient so the net effect is that the surface of the water if now in motion relative to the ship, meaning the ship can cause a smaller velocity profile of the water.