Sea currents flow from East to West around the world. That means it is easier to travel west over the water. This equates to twice the cost to go East.
Water that is heated at the equator of the world flow toward the poles of the world. This equates to an additional 25% cost to travel toward the equator.
The rotation and pull on the water (tides) by the gas giant causes the water to flow from east to west.
Water that is heated at the poles rise in the ocean and flows toward the poles. When this flow reaches the poles, the water is cooled, and it sinks. This causes water on the surface of the ocean to flow toward the poles and water deep in the ocean to flow toward the equator.