At night, we would have the north star. At day, we would have the sun. While the sun moves quite fast across the sky, a clock would not really need to be very accurate (and it can be ) to get indication of where north and south are. Your typical contemporary decorative hourglass can be expected to be accurate withing +/- 10%. But there were also precision hourglasses: "Not until the 18th century did <PERSON> and his son <PERSON>, come up with a marine chronometer that significantly improved on the stability of the hourglass at sea. Taking elements from the design logic behind the hourglass, they made a marine chronometer in 1761 that was able to accurately measure the journey from England to Jamaica accurate within five seconds."
Instead of "the Earth has no magnetic field and th eMoon has" (basically making life on Earth impossible, but maybe possible on the Moon...), a better premise would have been to have no ferromagnetic material available for navigation purposes. 840
Like some kind of rust-germs, which "eat" (degrade) ferromagnit stuff, or ferromagnetism is a "royal" privilege and only kings are allowed to own ferromagnetic materials (and every owner of an iron rod permanently risks receiving a death sentence in case this rod gets hit by a lightning or such).
To find north from the apparent sun's position:
You also need to roughly know your location on Earth and the current date. For any date and location, you can compute the apparent position of the sun in the sky for any daytime time. Or record the positions over the course of half a year for a certain location and use a table.
A trivial case: at local noon, the sun is in the south (assuming you are on the northern hemisphere). That is also when it has the highest elevation (that was used to find out the a ship's current latitude). Of course, finding the north direction just once a day is not sufficient. But with the pre-computed/pre-recorded table of sun positions, the apparent angle between the sun and north can is known for any daytime time.
Actually a good point: maybe the lack of a magnetic indicator for the north direction would have sped up the development of the marine chronograph. 393