As far as I understand it, a "water cream" is just a water based moisturizer without oils which is possible because of advancements in technology. They are definitely here to stay! As you say, they are perfect for oily skin or when you just don't need something heavy like during the summer.
I'm interested to hear which moisturizer it is that have become your new favorite! Personally I'm about to try the "Neutrogena Hydroboost Gel Cream Moisturiser" which I've heard a lot of good things about. My current moisturizers feel a bit too heavy during the summer.