I have used black gaffesr tape and it has worked best for me. I found electrical tape to be a little more reflective in certain lighting conditions, especially when using a surf housing, and I am above water shooting surfers.
Yes, gaffer's tape does not leave a residue when removed and sticks very good. No reflective etc. I use it on camera and when shooting 4x5t to attach filters to the back of lenses.
I have the Nauticam EM-5 Mark I kit and recently purchased the EM-1 Mark II camera (currently $1500 USD). I hope to get the housing before our fall trip to Indonesia.
The EM-1 II is super fast and has 4K video. If you go from the EM-5 I to II you will save beaucoup coins (including very fast and expensive cards) over the EM-1, but I think you would a kit that would be top notch for the future. Besides, the EM-1 is a great topside camera (currently shooting nesting raptors with a 300 mm lens).