Wash and pat dry the lemons, then cut them into quarters, removing any seeds as you go.
Place the lemons, water, and sugar in a large saucepan, ensuring that the lemons are fully submerged in the water.
Bring the mixture to a boil over medium/high heat, then reduce the heat to medium or medium/low and let it simmer for 30 minutes.
Add the sugar to the saucepan and stir until it has completely dissolved.
Continue to simmer the marmalade, uncovered, for about 1 to 1.5 hours, or until the lemons are soft and sweet.
Ladle the marmalade into clean glass jars, leaving some headspace for expansion.
Seal the jars and let the marmalade cool before storing it in the refrigerator.