Nick,

I think my longest day ever was 18 miles, not 20 but it was in a full wetsuit. I only paddle in a wetsuit. As do all my paddling buddies.

From my earliest days on the water I was taught that you must dress for the water temperature, not the air temperature.

I have canoed week long flat water and white water trips on all the big rivers in northern Maine in the spring (April/May to catch the spring gush but before the bugs come out) which often means freezing temperatures, dodging ice flows, snow squalls, etc.

I spend a good portion of my summers on multi day kayak trips on the coast of Maine and as you know Nick, that means anything from cold rain to hot and humid.

I always wear some type of wetsuit, every time.
I have a hydro skin Little John, a 3/2 shorty and a 5/4 steamer.

As far as their suitability for aerobic sports, (and let's keep it real here; flat water paddling, while not exactly pizza and beer, isn't really all that aerobic either) if they work for divers, surfers and triathletes I think your average sea kayaker would find them quite adequate.

People that live south of here may find this hard to believe or think this is overkill but it's not. We lose a few people here every year. Usually visitors from away but occasionally locals (that should know better). The typical scenario is they go out for a paddle on a warm summer day wearing shorts and a t shirt, get separated from their boat somehow, and succumb to hypothermia very quickly.

I lost a good friend and neighbor couple years ago, a Doctor actually. Obviously a very intelligent guy, very cautious, a life long water man. He died of exposure when he got separated from his boat in what is actually a pretty busy area close to shore. No one saw or heard him. It was almost 10 years to the day after his son died in a similar situation right near where we live.

If you are going to be out in water colder than 65F you must consider the possibility of hypothermia. Your core temperature only needs to drop 3 degrees for symptoms to begin.