Water temp in the Gulf the week Katrina hit - 98 degrees. Water temp off NYC at the height of summer - 72 degrees. What does a Cat 1 need to become a Cat 3-5....warm water.
Water tempretures are currently in the 80's as far north as Va. (I think 81 degrees is the minimum tempreture needed for a hurricane to form, but the storm can sustain itself at lower temps.) As hurricanes hit cooler water, they tend to speed up, which means you can have to consider both wind speed and the speed of the hurrincane itself.
Hurricanes threatening New York and New England gain their strength further south. As they begin to move north they pick up speed. They spend very little time over the cooler waters so they do not weaken that much. The waters a few hundred miles south of Long Island are still in the upper 70's. A storm moving north at 30mph is only going to have a few hours of weakening before landfall.
The water temps in the Gulf of Mexico were not 98 degrees before Katrina hit. They were between 86 and 89 degrees.