The water will probably be in the mid 60s (Fahrenheit) during June, the beginning of July and September, but in the 70s (Fahrenheit) during the end of July and August.
View a water temp graph for a full year (10/01/2004 - 10/01/2005) at the Battery, the southern tip of Manhattan.
View a water temp graph for the 2005 season (05/01/2005 - 09/30/2005) at the Battery.
View water levels, temps and other data from automated NOAA buoys around the US, including the Battery.
Convert Fahrenheit to Celcius for all of you worldly folks out there.
AccuWeather's five-day Weather Forecast - View the weather prediction for the next few days.
National Oceanic & Atmospheric Administration - This is a government site with good weather information.