FINA Rule WP 5.4
Players shall wear non-transparent costumes or costumes with a separate undergarment and before taking part in a game shall remove any articles likely to cause injury.
What this means is that although we have established some guidelines as to what is acceptable, the referee still has total discretion as to what may be allowed during a game and should inspect all players prior to the start of any game for those articles. We all know that jewelry, (including body piercings), wrist bands, and the like must be removed before play, not only to prevent injury to the wearer, but to their teammates and opponents as well.
It gets a little trickier with some other "medical" equipment such as prescription eyewear, splints, braces and protective face masks.
The guidelines for such devices are relatively simple and are along the same lines as the most recent FINA interpretations.
Articles (ie. facemasks and goggles) must be flexible with no hard or sharp edges that could cause injury to the wearer or others in the pool.
Finger splints must be padded and covered with no hard or sharp edges.
Player safety is first and foremost. If you intend, or are required, to wear a protective device it is recommended you get it checked out by the head referee in your zone prior to any competition to avoid not being allowed to wear that particular device in a game.