Differing submarine patrol areas strung along China's First Island Chain. Japan's submarine patrol area may be from Kyushu, along the Ryukyu island chain (which includes Okinawa), south to Taiwan, then across the Bashi Channel (Luzon Strait) down to Luzon Island, Philippines.
Part of the "First Island Chain" is what Japan calls the Senkaku Islands. A potential China-Japan-(maybe)Taiwan flashpoint due to undersea oil deposits. These deposits may become economically extractable as oil prices rise and technology permits.
Tetsuo Kotani, U.S.-Japan Allied Maritime Strategy: Balancing the Rise of Maritime China, (Strategic Japan, Japan Chair) Center for Strategic & International Studies (CSIS), Washington DC, April 2014, pp. 1-15. http://csis.org/files/publication/140422_Kotani_USJapanAlliance.pdf wrote on page 12:
"In addition, the submarine fleet will be increased from 16 to 22. Due to the lack of Chinese ASW capabilities, the expansion of the submarine fleet enhances sea-denial capability vis-à-vis the PLAN. To patrol the waters along southwestern Japan, it is estimated that at least eight submarines are necessary (six for the Okinawa island chain [Japan to Taiwan] and two for the Bashi Channel [also called (Luzon Strait) Taiwan to Luzon island, Philippines]. Typically, a [submarine] requires two backups for training and maintenance. Thus a submarine fleet of 24 is ideal, but a fleet of 22 provides more operational flexibility than the current fleet of 16.  On the other hand, for the effective use of the reinforced submarine fleet, the JMSDF needs to recruit and train more submariners."
 = [retired Vice Admiral] Masao Kobayashi, “Sensuikan 22 sekitaiseino Kaijoboei” [Maritime Defense under a 22-Submarine Force], Gunji Kenkyu [Japan Military Review], December 2011. http://gunken.jp/blog/archives/2011/11/10_0000.php
Page 1, 2nd paragraph indicates Japan's submarine deployments may, in part, respond to "Beijing’s attempts to deny access by other maritime powers to its Near Seas (the Yellow Sea and the East and South China Seas), which are enclosed by the first island chain (a chain of islands from Kyushu, Okinawa, to Taiwan and Borneo)."
Japanese strategy using all forces, including submarines, is partly to provide a blockading force (in time of conflict) to keep China forces and trade bottled up in China's near seas. Japan, its SSK owning allies, and the US SSN force could also block Chinese naval vessels and supplies (such as oil) from reaching China.
As the CSIS excerpt above indicates Japan's main submarine patrol area would be from the the southern home island of Kyushu, along the Ryukyu island chain (which includes Okinawa), south to Taiwan, then across the Bashi Channel (Luzon Strait) down to Luzon Island, Philippines.
Countries in the region may make frequent use of undersea arrays (along with future use of UUVs (including wavegliders)), particularly in narrow straits and harbour mouths. Such use would diminish reliance on virtually stationary submarines guarding critical straits and harbour mouths.
South Korea would monitor movements in all the congested seas and straits near South Korea. In those seas are threats or possible competitors North Korea, China, Russia and Japan itself.
The US with its wide ranging SSNs may guard some straits and narrows but the speed and range of its SSNs allow open ocean (blue water) coverage throughout the Pacific, Indian Oceans and under receding Arctic ice to the Atlantic Ocean. US SSNs can act as backup to the SSK navies (Japan, South Korea, Singapore, Australia) particularly against Russian and Chinese SSNs. All of the allies would directly or indirectly (via the US as the common ally) work together under the blanket term SeaWeb.
Naturally submarines are not the only blockers or monitors and don't work alone. Other platforms (surface warships, surveillance satellites, UAVs, UUVs, patrol aircraft including helicopters, land based missiles and ground stations (deploying radar, intercept and other arrays)) can work with submarines in the whole defence mix.