You’re absolutely correct about the topography. I’m not sure that jettisoning the bomb at sea would have been as ‘safe’ relatively speaking as later “broken arrows”.
The Savannah bomb was the result of a collision between a bomber & a SAC fighter (they had escort fighters back then, though I don’t think the planes were operating on the same mission). It’s DEEP in the mud of the Savannah river estuary.
Thankfully nobody had to make that call.
I think with the Savannah 'broken arrow' it's best to let sleeping bombs lie. We have lots of friends and family there, and nobody seems too exercised about it other than the envirowackos, the antinukes, and the newspapers.