I am a specialist AND a generalist. There are deep wreck and big animal wide angle dives where I live (South Florida), macro specialty havens like the Blue Heron Bridge, but also beautiful reefs for wide angle scenics that also are loaded with great macro opportunities. The 12-50 is a superb lens for the latter type of diving. I have ordered the Nauticam port/gear despite the cost because I think nothing else out there can touch it, in any format. At 12-18mm you have plenty of wide angle opportunities. As you zoom in, the focal lengths are excellent for fish portraits and head shots of larger fish. Close focus is good even in normal zoom mode.. This, of course, can be said of some other lenses in the 24-100 range (ff equiv). However, it is the macro mode that makes this lens really stand out. It is a "true" macro mode. The area covered in macro is roughly the area of a 35mm slide. While that is larger than 1/1 in the 4/3 format, it is roughly equal to 1/1 in 35mm, with no adapters needed. This is truly a remarkable capability. Deep closeups of feather dusters, head shots of jawfish, full frame blennies, all sorts of shrimp and arrow crabs are shootable in extreme closeup with very good quality. If you add a +5 or +10 diopter to the lens, you are now in ultra-macro territory, and with decent working distance (the macro mode focal length is 85mm ff equiv). I have not investigated, but plan to investigate, whether a wet mount wide angle adapter would also work to add wide angle reach if I cannot carry my wide angle lens/port for some reason.. The port is designed with the lens front element very close, and the lens does not change length when zooming, so this might be very feasable.
You can switch between all of these capabilities in mere seconds with the Nauticam port.
I also have the 7-14mm with dome, and the 60mm macro which also works in the 12-50 port. These are superlative specialty lenses that exceed the 12-50 in their respective areas.
BUT, If I could take only one lens? (for example a business trip where I might squeeze in a dive or two as opposed to a decicated trip), that is an absolute no-brainer for the 12-50.
Go for it!