During frostbiting I have been slowed by ice floes. In other words, the water temp must have been 32 degrees F (or less). I am sure other frostbiters have similar experiences. And yes, a dry suit is pretty much a must.
Never do it by myself. Even wet and dry suits have time limits once in the water. Probably best to stick
with sailing events where rescue personal are on hand. Seem to loose people here in Michigan each year
who fall out of boats in cold water. Been in freezing water once and can tell you the shock can cause muscles
to freeze. Can lead to cardiac arrest in some cases. If you can get through the first minute of cold shock you
may be good for another 30 minutes if wearing a life vest. Maybe someone here can explain the time limits involved in using a
dry suit vs. water temp. If I remember correctly a survival suit (Gumby) is good for about 45 minutes in freezing water
however it depends a lot on the person.
I have great respect for cold water. I had one encounter with hypothermia in my 20s when I was crewing on a Lightning in early May. The air temp was almost 80 but the water temp was about 50. We were fouled at a mark and capsized trying to avoid a collision and ended up sitting in the swamped cockpit for an hour waiting to be towed back to to the sailing club.
At the one hour point, I had little or no hand grip ability and could feel my mental state going downhill. Even with dry clothes and a warm shower, it took a few hours to feel normal again. Wet and dry suits help, but I avoid sailing in cold weather conditions even when the air temp is warm.