Score the peel of each lemon into four lengthwise sections; remove the peels with your fingers. Using a sharp knife, scrape off the white portions of peels; discard. Cut peels into thin strips.
In a large saucepan, combine peel strips the water, and baking soda. Bring to boiling; reduce heat. Simmer, covered, for 20 minutes. Do not drain. Section lemons, reserving juice; discard seeds. Add lemon sections and juice to peel-strips mixture. Stir in crushed strawberries. Return to boiling; reduce heat. Simmer, covered, for 10 minutes (should have about 3 cups).
In an 8- to 10-quart heavy pot, combine lemon-strawberry mixture and sugar. Bring to a full rolling boil, stirring constantly. Quickly stir in pectin. Return to a full rolling boil, stirring constantly. Boil hard for 1 minute, stirring constantly. Remove from heat. Quickly skim off foam with a metal spoon.
Ladle hot marmalade into hot, sterilized half-pint canning jars, leaving a 1/4-inch headspace. Wipe jar rims; adjust lids.
Process filled jars in a boiling-water canner for 5 minutes (start timing when water returns to boiling). Remove jars from canner; cool on wire racks. Allow to set at room temperature for 2 weeks before serving.