I have EM5 and Hero 3+ Black. I have tried this combo many times. It's most useful when I am shooting macro and something larger comes by. I have to warn you though, even though EM5's video capability is sub par due to very low bit rate, the color rendition and optical resolution is still way better than the GoPro because of GoPro's small sensor.
The bottom line is, GoPro needs light and a lot of it. If you are just using the GoPro on the side, you may not have lights properly done to accommodate it and the footage will suck. I have hours and hours of crappy quality video that have interesting scenes but too poor to produce a movie with them.
I am in Taiwan, mostly shore diving from sites around Kenting National Park. The visibility is around 10-15m when better, and down to to 5m when worse. 1m when really bad To find those shrimps, just stare at every moving ball of weed on the bottom and look for one that don't move like the rest.
In the last shot, there are a lot of particles lit up by the strobe in the water. It's not terrible, but definitely not a magazine photo. Visibility was about 15m that day. Taiwan has a lot of cool macro things to see. To be honest I'd say if you enjoy both land and underwater beauty, Taiwan is the place to visit. There are other places that are really good for diving, but nothing much else to see on land. I'd probably get bored easily.
I want to share some photos taken with the 12-50mm lens too. It's quite capable and can "get the shot". I am not in a place where visibility is good for wide-angle shots, but the macro side definitely does its trick.
Green algae shrimp. This was about 1cm only so this shot was cropped to 100% of pixel.
Sponge shrimp. This was slightly larger so I was able to fill 2/3 of the frame with it.
This mantis shrimp was about 10" long, I shot it on macro mode.
Wide-angle at 12mm.
Another wide-angle shot at 12mm. As you can see the visibility wasn't good and I probably should had turned off the strobe on the left.
From what I could tell, this lens with a flat port isn't sharp at corners and especially in wide-angle mode (understandably so). However, I think this would be a great travel lens when I don't know what I am going to encounter.