Lottie,

I have a neoprene vest that I bring with me to the tropics. I also bring a raincoat to keep the wind out even in SE Asia or the Carib. The thermos is a good idea, but I'd rather drink it during my SI than dump it in my wetsuit.

If you don't mind hoods, it would be the most effective. & I'm always freezing no matter where in the world I dive. Heck, I can even get cold in a hot tub