Halve lemons and squeeze out all the juice.
Tie seeds and pithy portion of rind together in a muslin or cheesecloth bag.
Cut remaining rind into thin strips.
Combine water and rind in a large saucepan over low heat.
Add muslin bag containing seeds and pith and continue cooking over medium heat until strips of rind are very tender, about 40 minutes.
Add sugar and return to a boil over high heat, stirring only until sugar dissolves.
Boil rapidly until marmalade reaches setting point about 20 minutes.
Allow marmalade to cool 10 minutes before packing into jars.