Shorts and a tee shirt but there is no way I would seal myself up in a airtight bag for water that warm, I would at most use a 3mil for the lower 70's and 1 mil at the upper range.
As pointed out in most of these "what should I wear" threads, cold tolerance is such an individual thing. Like @Rainer, I would freeze in what you wear. That's why I tried to give some examples of what is comfortable for me, and hope people can extrapolate some suggestions from that.
Diving steel doubles in this instance, so the drysuit is for redundant buoyancy (and, well, just to practice diving a drysuit and doubles). If I were diving a wetsuit in 75-78F water, I would use my 5mm and might even bring along a 3 mm hooded vest to layer in case the cold started setting in after a few dives. When diving wet, even on a summer day, a windy/cloudy surface interval can mean the difference between warming back up before the next dive and having to start the next dive already chilled. A drysuit in those conditions could be an interesting experience for me.