One option for watercrafts with an outboard or sterndrive is to tilt the engine upward and clear the line that way. Always make sure you have another person aboard to craft to watch and help if needed. If you lift the engine, and still cannot reach the line, try using a boat hook for extra reach.
Another option is to get wet and go in the water to untangle the propeller. This method will only work in certain conditions – when the water is calm and clear and there is another person watching from the vessel. In addition, remember to always wear a personal flotation device when submerging yourself in the water. Don’t bite off more than you can chew. If you do not feel confident that you can get the task done, try a different option.
Lastly, for watercraft with an inboard engine, your best option is to find a loose end of the line. Most of the time you will find it trailing in the water. Grab it with a boat hook. Have someone hold the line, retaining tension. Then have another person hand-turn the propeller until the line is free or until you have a sufficient angle to cut the line.