Macro lenses on GoPro don't in effect really magnify UW. What they allow you to do in this case is allow the camera to get closer to the subject while staying in focus. The better the lens, the closer with less optical distortions. Cheap ones will bring your minimum focal length from 12" to around 8" to subject. Good lenses as close as 4".
Yep, the a key question to ask is what do you intend on filming with the macro lens ? Will the subject let you stick the GoPro 4"-8" from it's face without swimming away and how still can you hold the camera ?