I don't have RyanT's knowledge but will say that any kind of swimming is probably good for most muscles. I know that to score well (well a "3", which I think is like under 13 minutes) on the DM 400 swim I had to train by doing months of weekly pool laps. I couldn't score well my first try despite a fairly rigorous bit of training including running-- but no swimming.
Since you use your legs for scuba, swimming makes sense (even the arm strokes, as you have to life heavy equipment and may use arms during scuba for very specific reasons).
But, since that DM test 10 years ago I have done zero swimming and noticed no problems diving, or back when I was assisting on courses.