3.5 mill for water in the 40's is fine, although it may take some getting used to it. when I lived in Virginia, I surfed all winter with a 4 mill wetsuit with water temps down in the upper 30's. I float tube and duck hunt with rubber waders and plenty of polypro underneath, and I am fine.
I was thinking of getting into float fishing this year as well, but I think Ill wait till its a little warmer! Any suggestions on what would be a good starter tube that wouldnt set me back to much doe?