My father was also on Okinawa. In the army, not the marines. Part of the invasion force.
This looks interesting. I just found it.
http://www.secondworldwar.org.uk/downfall.html
My dad was in the army and part of the invasion force. He told us (his nine kids) on numerous occasions that it is likely none of us would be here if not for the bomb (and the will to use it)