Do you wear a BCD with a dry suit?
Also question is, what do you wear under a dry suit?
Layer The Clothing The best clothes to wear underneath a drysuit is multiple fleece and synthetic layers. The temperature of the water temperature will determine the amount of clothes needed, not the air temperature. Start with a thin, short sleeve synthetic top and add enough layers of fleece to keep you warm.
Secondly, does a dry suit keep you afloat? If immersion is likely to be of a short duration, in water not too cold, flotation suits can do a good job but cold water hitting the body can kill extremely quickly. A dry suit keeps it off you.
Herein, how cold can you dive in a dry suit?
60 degrees Fahrenheit
Can you dive without a BCD?
Divers have been known to use their drysuits as a primary buoyancy device. Sport and technical divers should never consider diving without a BCD that has an air cell. A BCD is just like any other 'tool for the job' in that you should pick the right BCD for the dive you are planning.