I think it was Hofstadter (sp?) who pointed out that when we say "I can do anything I want", although we usually mean that our options are almost umlimited, in fact we're highlighting a severe constraint on our options - namely that we can ONLY do those things we want to do - in fact, in any situation, we can only do the ONE think we MOST want to do (consistent with physical laws, etc).
Sure, you can do anything you want to... but you can't control what you want!
I think the use of the bombs is probably the only think that has PREVENTED their use ever since... without an object lesson in just how powerful they are, no one would belive it. And no, blowing up a bunch of fish probably wouldn't have had the same impact.
Besides, if the japanese or germans or russians had acquired nuclear weapons first, does anyone seriously doubt that they'd have used them? The question isn't whether or not nuclear weapons would be used - it's who would use them. Me, I'm 110% OK with the outcome. No moral qualms whatsoever, and if I were in truman's shoes (with or without knowledge of the last 63 years) I'd do exactly what he did without a moment's hesitation, or a moment of second-guessing myself afterwards.
That's just me, your mileage may vary. But if it varies very much, I hope I never have to depend on you to defend anything important.