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):

water lemon
    n 1: the edible yellow fruit of the Jamaica honeysuckle [syn:
         bell apple, sweet cup, water lemon, yellow
         granadilla]