How speed is measured in ships?

The question can be understood in to different ways:

1. What is the standard measurement unit at sea?

This would be the knot, which is equal to 1 nautical mile per hour. A nautical mile is about 1.8 km.

2. In a ship, how navigators can tell how fast is the ship going?

There are a few ways to do that. One is to plot your position in a chart, maintain course and speed and after a while plot again the position, knowing time elapsed and distance traveled, you can then calculate average speed. Another one is by using a device called a speed log (there are several types). A third one, is to get a readout from your GPS receiver's display.