I wear my favoured sports bra (a Shock Absorber) and a pair of Under Armour Core Shorts as my "undies" under a wetsuit or drysuit. I actually wear the same sports bras and shorts running, so I have dedicated two sets to diving, and two sets to running and marked them.
I am busty (32DD) and most swimsuits, one piece or two piece, just do not feel supportive enough of my bust even if squashed under a wetsuit. I just find the core shorts super comfortable. I am not a very modest person, but both also provide enough coverage that I am not worried my breasts are going to pop out while undressing, or my butt cheeks won't be hanging out either!
Sometimes in cooler water I might add a rashguard or dive skin under my wetsuit, but I have a merino-lined wetsuit which actually feels quite comfortable and smooth against the skin and so forth so it is not really necessary.
You just need to find the right BC for you if you have a larger bust. I have a Soul i3 which has swivel buckles at the shoulder straps. As a result, they just naturally swivel to the sides and do not run over my breasts at all. The chest strap is also very adjustable vertically, so if you DO choose to use it, you can move it up or down to accomodate your chest and/or your drysuit valve.