Open water swimming in cold waters...

You may want to use earplugs. I also recommend getting a neoprene swim cap or at least double up on the swim caps. You lose a significant amount of body heat through the head. If you can keep some of the heat from leaving the head, the rest of the body handles the cold better. I have a full sleeved wetsuit so I can say one way or the other about using UA warm weather shirt, but don't think they are designed to work totally emersed in water.

The last couple of seasons I've done an early season race where water temps were in the low 50's.  The first couple of minutes in the water was pretty cold but then I was fine with just a regular cap and my wetsuit.

However, this is going to be very individual and based on your ability to tolerate cold.

Ear plugs are great if you have them as it can help prevent dizziness.

Shane

Had my first OWS in Ontario today and my full suit kept me perfect.   I do not know about a half though?  Brrr

Had my first OWS in Ontario today and my full suit kept me perfect.   I do not know about a half though?  Brrr

How long do you plan to swim and how often will you be in water this cold? 

One time, 10-15 minutes?  Double up on swim caps and put in earplugs.  You might check whether petroleum jelly (Vasoline) is a problem for your wetsuit, and if not, you might smear some on your neck (lots of blood vessels), hands and feet (lots of surface area).  (I think long distance swimmers have been known to use shortening on their skin instead of petroleum jelly when swimming across the English Channel. )  For a short, one-time swim I wouldn't invest a lot, but preventing the dizziness/headache can be worth a few bucks. 

Regular or 30+ minutes in those temps?  Neoprene booties, hood, and possibly even gloves if you're particularly susceptible to cold. 

The underarmour might prevent some chafing, but won't do much of anything to hold in heat because it doesn't stop the water. 

Wetsuits work because they let some water in and then hold it in and your body warms it up.  A UA shirt will not do this so it won't help keep you warm.  I would try it with just the wetsuit and see how you feel.  Don't give up if you are freezing after a couple minutes.  Like I said, the suit is supposed to let some water in but your body will warm it up and, as long as the suit is not to loose, will keep it there and you'll start to feel warmer.  

This is absolutely true.  In fact i now ski naked except for three hats.