If my coolant gage rises and then my car overheats, what is this indicative of?

Take it to a auto repair shop and get it checked! No way we can tell you what that is.

WATER is NOT what the owners manual says to use. It boils WAY to easy. Hopefully your mechanic is told this info, and any other symptoms , for correct diagnosis. From your description , I wouldn't be suprised if it's blown headgaskets and or cracked cyl. head(s).Good luck.