EAST ISLIP - Suffolk health officials once again closed Field 6 at Heckscher State Park to swimmers today because of the presence of brown water. Swimming has been banned in the area on and off for about two weeks.Health officials say tests have shown that bacteria levels in the water are too high for people to swim. It's believed the reason for the elevated levels is the presence of marine creatures called sea squirts."It's our understanding that the sea squirt is not harmful, and the best course of action is to take a shower and wash afterwards," says George Gorman, of the New York State Parks Department. The highest bacteria levels are near the shoreline, according to experts.