It could be helpful if you gave some more infos: Where do you dive most of the time - tropical/clear water or in areas with limited visibility? Only on holiday or every other weekend? (Analogy alert: I need a new car, which one could you recommend?)
That said, ask three people and get four opinions...
As troporobo said, the 12-50mm is a versatile lens - this is the first lens I would buy, together with the Nauticam port. Good for Macro, good for wide angle,
If you are a "macro guy", the second lens would be the 60mm macro from Olympus (or maybe a 45/30mm macro if most of your dives are in murky waters). If you are more into wide angle, the Panasonic 7-14mm is a great lens, even for close-focus WA. And an acrylic port is "good enough" (unless you regularly publish in National Geographic).
A fisheye lens is definitively not a good lens to start with - IMHO!
Of course you could buy the the Oly "Pro"-line lenses, but do they give you a guarantee for better pics? Nope.
A good way to spend the money you save with cheaper lenses is to join an UW-photo workshop. You come from a compact camera, your EM5 will give you much more options for better pics - but also much more options to screw things up... A workshop might be a good investment. The key factor for good pics is behind the camera.
p.s.: Here in the forum there is an old thread about UW settings for the E-M5.
Or look here: http://www.uwphotogr...rwater-settings
- Travi the Diver likes this