Drysuits are PPE for much of my work, which entails a fair bit of wading around in salt and freshwater, and consequently I have spent far too much time encased in rubber...
In my experience, there tends to be two broad types; the neoprene drysuits, which are fantastically warm, very flexible and complete overkill unless you expect to be in the water at some point. If you exert yourself in one of these you will be as wet as if you had gone in without it. That isn't the point though if you want it as safety gear, warm is always better then cold...
The neoprene seals however are worth considering. They can be fitted on teh other types of suit (See below) and are much less fragile than the latex ones. If you use cold cream or KY Jelly(!!) then they remain comfortable and last for ever.
Then there are the more common membrane type suits. These are great and there are loads to choose from. If you want a lightweight suit then there are all of the yachtie ones which you have probably already seen, or if you need something more industrial, there are several manufacturers out there. See links:
or (and annoyingly he hasn't a website, but used to do some very tough suits)
Goods Yard, Station Road
DT6 4EW (Road Map)
Tel: 01308 427352
The previous posts that refer too wearing suitable clothes under the suit cannot be overstressed! Once the outer layer is wet it will wick heat away rapidly, wear a Polar Bear suit or Thermal underlayer.
I have tried several of the breathable suits (N.Diver, Typhoon, Hammond, Collins and Remar) but in my humble opinion they make little or no difference once you become remotely active. Drysuits keep the sea off and let you gently stew...
Finally, if you are buying a drysuit under layer, see if they can make up any thermal booties in the the same material, I got some 10 yrs ago and they are still going strong!