The DC600 doesn't have a viewfinder it has an LCD screen. Are you having a hard time seeing the LCD screen?
I don't think a black mask will help much - thats more important when using a camera with an actual viewfinder, such as a dSLR.
Are you looking at the LCD screen straight on, or at an angle? Can you replicate the same issue on land? Perhaps your vision correction is impairing your view. Can you see it out of the top 1/3 of your lenses? Does Sealife sell an LCD to shield the LCD? There might be some aftermarket LCD hoods. When I had my Oly, using the lens hood was usually better than not.
David
David