While Japan has a powerful navy, the asia pacific region is filled with powerful Navies, I wouldn't say Japan is the sole naval power in the region.
The PLAN (Chinese navy) is number 2 largest navy in the world, and while not as advanced as Japanese navy, a war between the two will have no clear winner. In fact, if you're Indian, you should not underestimate the PLAN for they will be your future main rival in the Indian Ocean.
Russia had a monster navy 25 years ago with 280+ submarines, 6 carriers, 80 destroyers+cruisers, 250+ frigates+corvettes, of course it is a shadow of its former self now, but its submarines are still some of the best in the world. In terms of total tonnage, it is still the 3rd largest in the world, of course, noone knows the true tonnage of the Russian Navy as many of its cold war relics might not be operational.
South Korea, with a small population of 50 million, has the 8th largest navy in the world, with some pretty high tech stuff too i may add.
Taiwan, with a population of only 22 million, has the 10th largest navy in the world. While it is not as advanced as China, Japan's, or SK's navy (no ageis destoyers or advanced submarines), it is still considered a middle powered navy with plenty of anti surface ship capabilities.
Vietnam, Indonesia, Singapore, and Thailand are also modest naval powers.