You can see the pump hanging off the stern next to the discharge tube. This model of bait tank is a "Pump Dragger". You toss the bilge pump over the side and drag it through the water. There is a small amount of additional drag doing this, but not enough to cause a significant amount of paddling difference. Newer styles use scuppers for water movement.

The elbow in the side of the bait tank is the inlet. Not visible in the picture is the discharge elbow. He has a short tube on it, and the elbow can be turned. If the tube is pointing straight up, the tank has the max amount of water in it. If the elbow is rotated to the side, the tube is lower and water runs into it and out the tank. It's a way of adjusting the water level.