I would say just a couple of smallish cubes and no more than 2-3 times a week. Melon is quite watery. Someone please correct me if I'm wrong on any of that.
Water isn't a bad thing, I think it's more of a too much water and not enough nutrition that's bad. Might give them diarhea. Kind of like iceberg lettuce. It's not really much more than water and sugar.
Honeydew is high in sugar. Feed no more than a couple times per week. They could each have a 3-4 small cubes with the rind. My pigs love the rinds almost more than the fruit itself.
With any food it comes down to variety and moderation for most things (except hay and water). If you gave your piggies only say carrots, pellets and watermelon everyday then that would be a problem. If you give them various veggies and then fruit in small amounts every few days it works out. The problems usually arise when their diets are restrictive. To much of any one thing (with a few exceptions like hay) is not a good way to go for piggies or people for that matter.