As TSandM said, the floatieness, or rather the ability for air to circulate the undersuit is important. Fleece tends to limit air movement, which allows air to pile up in odd areas (usually the legs for maximum inconvenience).
A diving undersuit - it need not be an expensive brand name - will allow the air to easily circulate to the dump valve while not blocking it. This makes diving much, much easier with a dry suit.
Also, there are nice features that make a lot of difference, such as elastic bands to keep the suit legs from riding up, neoprene wrist seals that hold the suit on your arms, and the aforementioned ability to move.
What to remember is that an undersuit needs some underwear. some sort of wicking t-shirt keeps the zipper from scratching your gut and your sweat away from yourself. Cotton T-shirts just soak up the sweat and are really not that great.
I wear a thinsulate undersuit, woolly socks, neoprene 'ninja'-style oversocks and a sports t-shirt.
Oh, and boxer shorts.
You can certainly dive without the speciality stuff - lacking a good deal, this is what I would suggest- but it is just so much easier, warmer and more comfortable, so, when the opportunity arises, get the undersuit. Just my opinion, based on experience and watching others
Gerbs
