Chinese boat was suspected, mainly because for the last decade, Chinese subs have increasingly been showing up close to Japan and South Korea as well.

猜测是一艘中国潜艇，主要地因为最近十年，中国潜艇日益在接近日本和韩国等海域出现。

youdao

Military analysts expect South Korea to spend more on conventional weapons designed to defend it against the North, including subs, destroyers, F-15 fighter jets and possibly F-35s.

WSJ: Asia's New Arms Race