After landing on the submarine, take out the two enemies on top, then disarm the bomb and enter the submarine.
In the first area, there will be four enemies on the top level and three on the bottom. After taking out the enemies, continue to the next area through the doorway on the bottom level. This area will contain 11 more enemies, most of which are on the bottom level. Continue through the door where the enemies keep entering from.
Go down the staircase and kill the 5 enemies, then continue forwards. After entering the reactor area (the yellow circle on the map), turn the valve and defeat the 6 incoming enemies. Continue up the ladder, turn around and kill the two enemies, as well as the three that drop down from a helicopter. Progress forwards and kill two enemies in the smoke. From here, kill the last four enemies that drop down from a helicopter to finish the level.
