It was 1 p.m. on Tuesday. DeThorne, head lifeguard at Schenectady’s football field-sized pool, had nine people on her watch. Gray skies and mild temperatures — the city’s summer oven was only 76 degrees — had persuaded some aquanauts to stay home. DeThorne, 20, who graduated from Schenectady High School in 2009 and will be a junior this fall at the University at Albany, has been watching people swim at Central Park for the past five summers. Nine people in the water is a small crowd. On some days, when it’s 90 degrees in the Capital Region, there might be a hundred people or more in the 900,000 gallons of cool blue.