Water is always pretty cold here. Everyone almost always has a long sleeve dry top. The only thing we change is the thickness of the thermal layer under the dry top. It can be a thin rash guard, or a thick thermal layer.
I don't think you need a full dry suit. They are a pain. If you have any type of synthetic material pants, that should be enough for you to survive a long swim.
For real cold water, you want a cap under your helmet and gloves or pogies. Go to the local kayak store for those.