How long is a piece of string?

When you leave the harbour you must do so in a straight line out in the lake for 300m. Nothing I have experience from since I park in the middle of Zürich city, where you have to navigate through swimmers, paddle boats, canoes, rowing boats, ferries, sail boats and other motor boats. The only time I think birds are clever, because they always take a turn when you come too close. How long then is 300m on water? Well, they tell you to look at the church clock. When the clock reaches the horizon it's 300m. At least in the home harbour of this particular boat. I guess with experience you can tell, but it ain't easy I can assure you. Why straight line? After all it's water. But apparently this is important. At least for the test :)