You can probably get it polished from someone like focal point in Colorado, cheaper than a new one. If you can feel the scratch with your fingernail then it is probably too deep to fix. There are people who think they can do this themselves but they are mostly wrong. I think focal point charges between $200 and $300 to polish such lenses, there are other companies that do it as well.
In my hands, the CMC (on the Oly 60 lens) is much better than the SMC and the Multiplier (better DOF and easier to focus) Still the working distance is pretty small. I love my 45 finder, all I ever use but there is a learning curve.
For a strobe with a GN of 20 (meters ISO 100) on land, I would estimate a GN of 10 (or less) underwater. If I want to shoot at f:11 then I can be around 0.9 meters or a bit less than 3 feet away. That is why (even with strobes) the rules about get closer and closer are what they are.