How can measure ship’s velocity?
(In that ship)

Keeping in mind that all velocities are relative...

Measuring the velocity relative to the water is pretty easy - use a device like like an impeller whose speed is calibrated against water flow. Or do like the old sailors did - through a buoy overboard with a long rope attached that has knots every 10 feet or so, and time how quickly the rope pays out (the speed of boats is often measured in "knots," precisley because they used this method).

To measure velocity against land is a bit trickier. You can use compass, sextant, and an accurate clock to plot your position on the globe, and then calculate the change of position each day, and hence your average velocity each day. Or if you have a fancy gyroscopic system that can accurately determine the acceleration of the boat starting from the time you leave the dock, and from that determine velocity (this is how navigational systems on boats and planes typically worked). Or you can use radio-based distance measuring equipment - you send out a ping and see how long for an answerting ping to come from responding equioment on shore. Do this with two or more stations at known positions and you can triangulate your position, and hence determine velocity. And of course these days the easiest solution is to use a GPS to very accurately determine your position, and hence velocity.