For what it's worth here are the general guidelines. Considering how tight the suits are, it seems like a swimmer could improve buoyancy by trapping some air into a suit.
The general swim wear rules from FINA (General Rule 5) state that swim suits, caps, and goggles:
* Must be of good moral taste
* Must not have any offensive markings or symbols
* Must not be transparent
* Must be approved by FINA
* Must be available to all competitors
* May be one or two piece
* May not extend past the ankle, wrist, or neck
* May not have additional parts like arm bands or leg bands (two pieces does not include these pieces!)
* May be excluded from a competition by the referee of that competition if the suit or body markings do not comply with the rules (this part of the rule includes body marking, like tattoos, too)
The approval process for swimsuits, caps, and goggles (Annex 11, FINA Requirements for Swimwear Approval) defines the steps a swimwear supplier must take to get their swim gear approved for use by FINA. These steps must be repeated every 4-years. These regulations also list more specifics on swimwear rules, including:
Swim Suits
* Male swimwear must be one piece, women's swimwear may be two pieces comprised of one piece for the upper body and one for the lower body
* Swimwear material must not endanger the health of the swimmer
* Swimwear material must be "regular" with no outstanding structure or shape, like scales, and nothing can be added to the fabric to form a structure. No scales, dots, etc. (looks like a lot of swimwear used at the last Olympics may not meet this rule - scales, trip wires, silicone dots, etc.)
* Swimwear may have paint or impregnated markings, but these may not exceed "usual" thicknesses
* Seams must be functional and must not form shapes beyond joing fabric together
* Laces can be used on swimwear, but only to hold the suit around the waist
