-- Last Updated: Mar-04-13 2:31 PM EST --
In the conditions specified in the OP.
I've gotten more concerned about overheating in hot weather than swimming as I've gotten older. To me, dress under any given conditions is a balancing judgment of several risk probabilities based on my personal paddling history all over North America for many decades.
During a solo week in Georgian Bay, where I was paddling about 18 miles a day, I wore basically lightweight summer shirts and pants, maybe shorts and t-shirts if it was in the 70's F or above. I always carry a towel and change of clothes.
I felt quite safe doing this, as in the all parts of Georgian Bay I paddled, I don't think I was ever more than than 50 yards from land. That's the appeal of the place -- a labyrinth of 30,000 islands. Plus, I was on an outrigger canoe, which is easy to climb back on for self-rescue, which hasn't ever been necessary for me in any canoe or kayak for 30 years.
If there are long crossings in waves or wind, I simply don't do them in any craft. I stay near shore where the scenery and safety are both much better.
If I were paddling the open ocean in those air-water temps I would probably wear my hydroskin wetsuit, but I don't like wearing wetsuits and have never really liked paddling in northern oceans, so I don't do it any more and the issue doesn't really arise.