The Collaborative International Dictionary of English v.0.48:

Water lemon \Wa"ter lem"on\ (Bot.)
   The edible fruit of two species of passion flower
   (Passiflora laurifolia, and Passiflora maliformis); -- so
   called in the West Indies.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Passiflora laurifolia
    n 1: West Indian passionflower; cultivated for its yellow edible
         fruit [syn: Jamaica honeysuckle, yellow granadilla,
         Passiflora laurifolia]