I will try to keep this brief, I recently returned from a weeks diving in the red sea and was my first extended go with new camera and housing (details below), the problem I had certain wildlife is easily spooked, no matter how slow I approached, minimal/no bubbles with the lens on the camera I could not get close up for some of the shots I would like to take.
Two specific occasions were: a large Napolean wrase was behind a pinnacle I swam around to get a photo, said wrase then proceded to swim around the pinnacle as soon as I got closer.
Second occasion was an amazing dive with 20+ hammerhead shark shoal (unsure of the plural) it was a surreal dive but looking back at the photos I would have like to have got a good close up (able to see denticles on skin would be good), in this situation the sharks were very curious but wary. Over the space of 4 dives gradually got closer but 2 metres was the limit they would swim off if we got any closer.
My camera is a Canon EOS M in a Nauticam Hosuing with a 22mm f/2 lens, it performed really well for the majority of the trip apart from close ups (bar lion fish and moray's which dont move away!). My choice now is one of the other EF-M mount lenses (18-55 f3.5/5.6 or 55-200 f4.5/6.3) or use the canon adaptor (already own), and fit any of canon's lenses.
I own a 600D and have a few lenses, above water this would not be an issue as I could try a lens and if I didnt get on with it sell it on or return if for minimal loss, the added complication being if i purchase a new port then it adds quickly to the cost of trials and are more difficult to sell on/return.
Any adivce or opinons welcome !
N.B. Current idea would be keep using the 22mm lens (with strobes I am saving for) most dives and then switch over to the longer lens for specific close up dives.