It's not realistic because generally, on Earth, you aren't crossing the Pacific ocean to get to that tiny little island, then crossing it again to get to another little island, and staying out in the ocean boating for days and days. Major areas of Earth aren't on islands; they're on the mainland!
When people see Pokemon, unless they've looked up on Bulbapedia about it being based off of Japan, they imagine an Earth similar to theirs filled with Pokemon. On Earth, we go on relaxing cruises, but how many people, other than sailors, spend a crapload of their lives on the water?
See? It's not realistic. We aren't talking about the percent of the earth's volume in land and water; we're talking about the reality of being on water for a crapload of time to travel to place to place. Maybe 400 years ago when there weren't planes to get around lol and we all know that Pokemon has the Bullet Train in Goldenrod, and the Speedboat from Bill, so why not use that to get to those pesky islands instead? In reality, would YOU be swimming or rowboating from island to island in Japan? You'd take a plane. Or an underwater train. Or a speedboat. You wouldn't be out there on a rowboat boating for three days, unless you're poor as Hell.
I certainly don't imagine Japan when I imagine Pokemon; Pokemon is its own world. It may have been influenced by the Japanese country, but heck, they could've nicked it to not have so much space BETWEEN the islands and perhaps make more routes on the islands themselves, rather than forcing us to use Surf the entire time.
