There are two important concepts with a wet suit, fit and the material thickness. The fit keeps water from circulating in and out of the suit. Material thickness is the insulation you need. The thicker the suit, the warmer it is IF there is no circulating water. The deeper you go, the thinner the suit becomes, so if you are going fairly deep (40-60 feet), and in cold water, you will want more than the semi-dry suit. You need layers, provided by the Farmer John bottoms and jacket. You may want to get a custom fitted wet suit. I understand that they are still made, but not as much as 20 years ago.
For a discussion, search for the thread on Clear Lake in this Exposure Suit topic area. I have a fairly extensive discussion of wet suits there.
SeaRat