Ehh, the fish is just one part of sushi. The rice is just as, if not more important, unless you're one of those sashimi-only weeaboo shits. The place I usually go to (and went to because this new place was goddamn shut down) has the best rice I've found. A lot of sushi fish is aged anyways. The reason proximity to a coastline leads to better sushi is because whoever is in charge with getting the restaurant's fish (assuming it's not farmed out to an intermediary supplier) can be on-site to see and hand-select their first choices of individual fish. All the retarded birth defect fish get sent to Iowa.
Edit: A secondary reason is just established talent. Big-name chefs flock to coastline areas for aforementioned reasons, fledglings go there to try and learn from and train with those chefs, etc. No one's going to Nebraska to learn shit. Luckily Vegas has a halfway decent scene despite being pretty far off the coast because of the high-class hotels buying out good chefs, but still not as great as when I was in Seattle.
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