In my blog I made a comparision of 30mm, 50mm and 100mm macro lenses taking the same subject at approx. the same magnification:
You will see that the DOF is the same in all 3 cases (at given F number) but the perspective is completely different. Check especially the relationship with the background (not just the background blur and the field of view, but how the close subject behind the main one looks).
They were taken not underwater but I hope it's helpful.
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