A PRECAUTIONARY BOIL WATER ADVISORY IS IN EFFECT FOR SOME SIESTA KEY RESIDENTS AFTER REPAIRS WERE MADE TO A WATER VALVE AT THE INTERSECTION OF MIDNIGHT PASS ROAD AND SARASEA CIRCLE.  ACCORDING TO SARASOTA COUNTY OFFICIALS, THE WATER WAS TURNED OFF AROUND 11:30 YESTERDAY MORNING FOR ABOUT TWO HOURS, AND IT AFFECTS 320 CUSTOMERS.  WATER SAMPLES HAVE BEEN PULLED, AND WHILE THE COUNTY HAS NO REASON TO BELIEVE THE DRINKING WATER WAS CONTAMINATED, FLORIDA LAW REQUIRES THAT DRINKING WATER PROVIDERS ISSUE PRECAUTIONARY BOIL WATER ADVISORIES TO CUSTOMERS AFFECTED BY PRESSURE LOSSES IN WATER MAINS.  CUSTOMERS SHOULD BOIL TAP WATER INTENDED FOR DRINKING, FOOD PREPARATION, WASHING FOOD UTENSILS, MAKING ICE, BRUSHING TEETH OR FIRST AID, MAKING SURE WATER IS BROUGHT TO A ROLLING BOIL FOR ONE MINUTE.  ONCE THE WATER SAMPLES ARE TESTED AND THE WATER IS DEEMED SAFE TO DRINK, CUSTOMERS WILL BE NOTIFIED BY AUTOMATED TELEPHONE.  AS AN ALTERNATIVE, BOTTLED WATER CAN BE USED.