Under Armor is almost a most. Its wicking properties will go a long way towards keeping you cool and comfortable. Trust me, it will get hot in your armor, I've been doing a Stormtrooper for 6 years now and under Armor, or even a cheaper knock off, is a very nice thing to have.
I would have to caution against the wetsuit idea, becuase they are often much thicker, and some are often actually designed to keep in body heat to prevent you from losing it into the water, so that isn't the best thing to have to prevent you from overheating.
My ultimate recommendation would be custom making an undersuit out of as lightweight of material as possible that resembles the actual in game undersuit, and wearing under armor underneath it. That way you get accuracy and keep your temperature down.