I can tell you my experience; I got certified last year and our final checkout/test dives kept getting pushed back because the hurricane and other storms kept ruining visibility every weekend we would be scheduled to go! So finally, we ended up diving in October. I had a 7mm wetsuit, 7mm boots, a 5mm or 7mm hood (I think it was 7) and 5mm gloves (you really can’t bend your fingers at ALL in 7mm gloves.) The water was about 52-53 degrees on the surface, and 20-30 feet down it was 50 degrees even.
When I first got into the water, I thought I was going to go into shock when the frigid water first raced up my spine (you know, when it first gets into your wetsuit?) Yeah, not fun! Then after being in the water for about 5 minutes, it was really hard and took a lot of concentration to resist/slow down the convulsions from being so cold. I truly don’t know how we got through 20 minutes and then another 20 minute dive, somehow we did it, but I had to mentally put my consciousness in another place, as though resisting torture. I don’t think you could pay me to dive in 50 degree water again without a dry suit.
So I think you want the water temp at the level most of your dive will be at, to be around 60 degrees at least. I spoke to my former dive instructor this past weekend and he told me he dove and it was 52 degrees (and that was the same place I did my checkout dives, so I am pretty sure he was 20-30 feet down again.)