As little as possible, particularly in cold water. I dive an open cell wetsuit where the primary warmth comes from the rubber being in direct contact with my skin - any fabric between me and the suit results in colder water.
So basically it goes like this:
Warm tropics - micro bikini, long sleeve rash guard top (to protect against jellyfish etc)
A little cooler - micro bikini, open cell wetsuit, long sleeve rash guard on top
Colder - bikini, long sleeve rash guard, 7mm semi-dry wetsuit
A lot colder - micro bikini, open cell wetsuit, long sleeve rash guard, 7mm semi-dry
And if it is even colder and everyone else is in a dry suit, that combo gets a 7mm hooded topper, gloves, etc. 15mm of neoprene is definitely not fun but it works.
So basically it goes like this:
Warm tropics - micro bikini, long sleeve rash guard top (to protect against jellyfish etc)
A little cooler - micro bikini, open cell wetsuit, long sleeve rash guard on top
Colder - bikini, long sleeve rash guard, 7mm semi-dry wetsuit
A lot colder - micro bikini, open cell wetsuit, long sleeve rash guard, 7mm semi-dry
And if it is even colder and everyone else is in a dry suit, that combo gets a 7mm hooded topper, gloves, etc. 15mm of neoprene is definitely not fun but it works.