The JTS engines seem to, for some reason, normally produce an hell of a lot of water vapour from the exhaust - it's normally very noticeable during the winter, stopped in traffic you'll easily see a wispy cloud coming from the exhaust - it looks exactly like the one coming out of your mouth. I have to say, however, that I've never seen it while moving.
That said, do your oil and coolant level drop at any rate? Besides the yearly services, I never ever had to top up either the coolant reservoir or the oil - they always stay at the same level they're filled up during service. If they drop noticeably, or one drops while the other rises, there might be a problem. You may also want to check either for contamination.