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I don't think the sea is a viable option. A ship is easy to spot, and an elven ship would be very conspicuous. The Corsairs would certainly intercept it, along with the winged Nazgul.
I think that part of it is simply literary adventure. And, we had to have an Evil that could be on par to Gandalf for that part of the story, which I am unsure would have been that plausible if they had gone another route.
And Moria is one of the best parts of the book.
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