Where I live it is about 250 feet above sea level and it was about 95F with 75% humidity, barometer 30.00in; the air had the same density of a cool day at 3000 ft. Your engine has less power because there is not enough oxygen to make max HP and there is not as much bite with the prop and lift with the wings. Humidity makes the air feel thick, but the water is displacing the oxygen content, so you feel thick air of the water, while you suffocate from lack of O2.