I usually just try and keep the dry suit wet when it is hot out. The evaporation seems to keep you mostly comfortable. If you can, Gore-Tex is the way to go when you get a dry suit.
I would try and avoid only wearing poly-pro and splash gear if there is any chance of falling in to very cold water especially when there is long swim consequences or you are responsible for other people. Then again I usually tend to sacrifice comfort for a bigger safety margin, you should do what ever you are comfortable with.
One other thought about dry suits in warm weather; even if you do have light layers on underneath and you have a longer swim you can get off the river and open the dry suit up and you will warm up much faster than if you are soaked.
Watch this for some sunny day cold water thought.