the water cooling does what it says, helps to cool the turbo, i wouldn't reccomend running a roller bearing turbo without a water feed but its not so bad on a conventional turbo, as said, just treat it with respect as you should do water cooled or not!
A fiesta with a T3 is a classic case, most have a S2 T3 with water cooling, i always reccomend leaving the water disconnected as otherwise it tends to make the car run to hot...
As for letting the car idle when finishing a journey, i normally do the last mile or so trying to heep the car off boost, then when stopped idle for 30 secs to a minute, if its been driven hard then raise the bonnet & let it idle for 5 minuted!