Use a bowline knot as they can be undone.
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And quite unsurprisingly, you can also undo a figure of eight knot. However, get a bowline wrongly tied, and I've seen plenty of them on other peoples boats, and it can fall apart remarkably easily.
As for getting the turns out of a coil of rope, this should be done with every coil, but some of the worst offenders are those who coil a rope incorrectly, for example from the free end towards the fixed, or around hand and elbow etc., and those who use it directly from a new coil.
One of the easiest ways of removing the turns is to pay the line out over the stern, and tow it for about ten minutes - just don't let it go by mistake, or run your engine astern at the same time, and only do it in deep water and when well away from all other boats. Then give it a good stretch with some weight on it overnight, or longer.