I am not sure if the 30mm macro lens makes sense underwater. Macro lenses are optimized for - guess what - macro. But with the 30mm lens subject distance definintively will be too close when used in macro mode. As you mentioned, difficult/impossible lighting, and of course only for static subjects.
Of course you can use the lens for subjects further away and have less water between the fish or whatever and your lens than with the 60mm lens - but why not use a zoom lens instead? Much more versatile.
- bmorescuba likes this