Using a paring knife, remove peel from grapefruit in large strips, taking care not to remove too much of the "pith" or white part. Thinly slice peel, to make 1 cup.
In a small saucepan, combine peel and water and bring to a boil over high heat. Reduce heat and simmer, covered, for 15-20 minutes. Set aside.
Remove and discard the remaining peel and pith from the grapefruit. Finely chop grapefruit, discarding seeds and any connective membranes.
Measure to make 3 cups, including juices.
In a dutch oven, combine grapefruit, cooked peel with liquid, and pomegranate juice. Bring to a full boil over high heat; reduce heat and simmer for 5 minutes, stirring occasionally.
Stir in pectin until dissolved. Bring to a full boil over high heat, stirring constantly.
Add sugar in a steady stream, stirring constantly. Return to a full boil, stirring constantly to dissolve sugar. Boil hard for 1 minute. Remove from heat and skim off any foam. Stir for 5 to 8 minutes to prevent floating rind.
Ladle into sterilized jars to within 1/4 inch of rim; wipe rims. Apply prepared lids and rings; tighten rings just until fingertip-tight.
Process jars in a boiling water canner for 10 minutes.
Transfer jars to a towel-lined surface and let rest at room temperature until set. Check seals; refrigerate any unsealed jars for up to 3 weeks. This marmalade is best served after resting for 1 week.
Used with permission from 250 Home Preserving Favorites