Are other fruits, like watermelon, oranges, and grapefruit a good source of water-borne helpful life forms?

This is very much the case as well. This is not species-specific with respect to the plants. It is a function of water content and its accessibility. Water can be extracted from cactus plants by doing a little work, so the juicy melons and grapes are a much more ready source of natural water, pound for pound, with little effort and will preserve these life forms during their natural useful lifespan once harvested.