I have read somewhere that you can add up the air and water temperature
to determine the degree of hypothermia hazard. What I don't remember is
the range of total temperature that was relatively safe vs. unsafe. I
did some google searching without finding what I was looking for. I did
find some survival time tables as a function of water temperature, and
one reference that said you should wear a wet suit if either the air or
water temperature is under 65 degrees F.

I am mostly a river canoe person, but I do get out on lakes from time to
time.

Yes, I know quite a bit about hypothermia, have read a lot about it,
experienced it, pulled a hypothermic swimmer out of the water (I still
tell that story 30 years later), etc. I wear a farmer john wetsuit when
I think I might swim. I do not paddle in the wintertime.

Anybody have a pointer to an article with rules of thumb?

Richard