A rescue mask for CPR starting coming out as part of the rescue back in the 1980's/1990's with the concerns regarding body fluid transmissions of diseases. The fear at the time was HIV/AIDS but other body fluid disease such as hepatitis B are actually easier to transmit than HIV. In general it is not a bad idea to have at least one in the first aid kit.
Recent American Red Cross guidelines have been changing to emphasize effective chest compressions over rescue breaths. It is currently acceptable that if a rescuer is unwilling, or untrained, chest compressions only are an acceptable alternative.
As far as putting a mask in your BC pocket, I'm not sure how much more that would help in a rescue. To have either rescue breaths and especially chest compressions the victim should be on a hard surface. In the water this is near impossible unless a back board is available. It would be better to get the victim either on shore or on the boat ASAP rather than try starting ineffective CPR while in the water.
Besides, if you are diving with a friend as your buddy you shouldn't have a huge concern regarding a body fluid disease. At least I hope not. All bets are off if you are with an instabuddy which is for practical purposes a complete stranger.