When Jonas gets the operational plan from the safe, it is classified "Top Secret-No FORN." "No FORN" stands for "No Foreign Nationals" meaning that the information in the plan cannot be shared with non-US personnel. He hands the plan to the Korean skipper for him to look over. Additionally, the cover of the plan is in Korean. A "No FORN" plan would only need to be in English since foreign nationals are not allowed access.
At 9:26, the submarine is already submerged. The depth is increased, but the captain and executive officer shout "dive," and an alarm is sounded. This command is only given in this manner when a sub is on the surface.
The South Korean submarine is described as being a Cold War-era vessel, yet remains submerged and on battery power for the entire mission. Diesel-electric submarines of that era had limited battery power and had to surface (or nearly so with a snorkel) to run the diesel engines, recharge the batteries, and exchange air.
Jonas consults his handheld GPS receiver and determines that the sub is 13 miles off course. The GPS signal can't be received underwater, much less inside of a steel can. Subs can only receive VLF (Very Low Frequency) signals underwater and then only an antenna outside of the sub's hull.