It is nearly inevitable that it will eventually happen. Sooner or later, anyone who participates in boat towed water sports will experience getting a knot in the rope. It can happen so fast that there's nothing you can do to stop it.
Often, you can look for loose sections of the knot and simply pull the knot apart. But, if you have used the rope to tow a skier while the knot is in place, the knot can become extremely tight and stubborn to get loose.
You will want to remove the knot as soon as possible. Continuing to use the rope with a jammed knot to tow a water-skier or wakeboarder will begin to weaken the rope around the knotted area. This could lead to the rope fraying and eventually breaking apart.
Here are a few options to untying those stubborn knots:
1) Firmly and rapidly twist back and forth the sections of the rope just outside the knotted area. At the same time push in on the knot to create slack.
2) Lightly tap on the knot with a hammer. This will often loosen the knotted area.
3) Find a blunt, pointed object such as ball point pen or a blunt spike. Place the knotted roped on a flat surface. Gently insert the tool's pointed tip into the rope and move the pen around in a wide circular motion. This can help loosen up the knot. Be careful not to tear any of the fibers in the rope.