Did just that and the hole is far too small, a diameter around 50 meters. (And I was highballing, the actual size would be less than that)
The crater shouldn't be any smaller than this. The explosion is bigger than the giant, which is 1000X bigger than Star. A 50 meter crater would be far smaller than the giant. Using the hole is pointless, but the ocean depth being used also seems incorrect. So I don't think it is possible to truly calc the size of the crater using that shot.
Best bet would be to just calc the explosion, and use the explosion formula. Though the current calc assumes SaS is All Might size, which I believe is incorrect, for safety I'd put her at 6 FT or 1.82 m. Doing that gets me 29.96 MT for the explosion, divided by 10 and we get 2.99 MT for a single missile.
Which is... Low 7-B. Consistent?