Yes you can mix heavy cream with water (or just use it straight) to make great flans & custards. Since a properly-cooked egg custard takes 45-60 minutes in a water bath in a 300 degree oven, I came up with a quick microwave custard that I often have for breakfast:

3 whole eggs
3-4T heavy cream
1/2 tsp vanilla
dash salt
sweetener to taste

Blend in Magic Bullet (tall cup & flattish blade), nuke on high for 60 seconds in the MB cup (uncovered); blend again, nuke for another 30-40 seconds, blend again and then let it sit on the counter to cook the rest of the way on its own. I eat it right out of the MB cup with a parfait spoon. If you don't have a Magic Bullet, a stick blender in a tall cup should work too. These times are for my microwave, so YMMV, but start low - you can always cook a little longer. I don't add any water, but if you do, extra seconds are in order. Finding the best timing & blending gives a nice smooth custard, but even my lumpy trials tasted great. This breakfast keeps me satisfied for 5-6 hrs.