My first layer under my trilam drysuit is the previously mentioned Polar sports bra and a pair of underwear. Not going to tell you what kind! Only PEARCE knows for sure. 
The next layer is entirely dependent on the temp of the water AT THE DEPTH I AM DIVING. Also, I tend to get hypothermic quickly, so I may wear more than someone else might.
For example, here is what I would wear at Vortex Springs FL, where the water temp is 69F, year round:
1st layer: already discussed.
2nd layer: long sleeved poly shirt (used by hikers for warmpth & moisture wicking ability), bicycle leggings, and moderately thick hiking socks.
3rd layer: sweatshirt.
If I was diving in colder water, I might wear a heavier sweatshirt, or put on a t shirt over the poly shirt. Similarly, I might add heavier bicycle leggings, like those I wear to cycle in the winter. For me the key is layering.
Meanwhile, I'd like to point out that at minimum I would wear a long sleeved t-shirt and thin leggings because of the possibility of suit pinch at the joints (knees & elbows). So even if I don't need the cloth for WARMPTH, I will certainly need it for protection. (Nothing like coming up after a dive to find you have big purply hickies in the back of the knees or elbows due to suit pinch.)
ut:
~SubMariner~
The next layer is entirely dependent on the temp of the water AT THE DEPTH I AM DIVING. Also, I tend to get hypothermic quickly, so I may wear more than someone else might.
For example, here is what I would wear at Vortex Springs FL, where the water temp is 69F, year round:
1st layer: already discussed.
2nd layer: long sleeved poly shirt (used by hikers for warmpth & moisture wicking ability), bicycle leggings, and moderately thick hiking socks.
3rd layer: sweatshirt.
If I was diving in colder water, I might wear a heavier sweatshirt, or put on a t shirt over the poly shirt. Similarly, I might add heavier bicycle leggings, like those I wear to cycle in the winter. For me the key is layering.
Meanwhile, I'd like to point out that at minimum I would wear a long sleeved t-shirt and thin leggings because of the possibility of suit pinch at the joints (knees & elbows). So even if I don't need the cloth for WARMPTH, I will certainly need it for protection. (Nothing like coming up after a dive to find you have big purply hickies in the back of the knees or elbows due to suit pinch.)
ut:~SubMariner~
