You can get hypothermia in water temperatures higher than 70F easily if you stay in long enough. 70 degree water is quite cold at least to me. I was at a beach in Florida a couple of weeks ago right about that temperature or a little warmer. I wanted to swim in the ocean but it was so cold all I could do was wade up to my knees for a few minutes. I certainly wouldn’t want to be stranded in it.
Hypothermia is caused when your core body temperature drops even a couple of degrees below normal. It doesn’t take very cold water to cause it if you stay in long enough (although that may mean many hours) because water is so heat conductive.
This article says that you can get hypothermia in water in the 80 degree range given enough time in the water.
“How cold does the water have to be to put a person at risk for hypothermia?
Even water temperatures as high as 75 and 80 degrees F (24 and 27 degrees C) can be dangerous, but it would most likely take much longer than 15 minutes to become debilitated. There is no set time for when hypothermia will set in, but generally the colder the water, the faster it happens.”