For a submarine to stay submerged, it must be neutrally buoyant, which means it is maintaining a careful balance between its weight and the buoyant force of water pushing it toward the surface.

Answer 1

Yes it is!

Just remember that it is based off of direction meaning if you are basing it off the direction of gravity the force is normal which means perpendicular to the surface of Earth. So the buoyant force is opposite direction of force of gravity which means that if it is staying in one place underwater the system F_net = 0 so force of gravity = buoyant force .

But.. In order for the submarine to go under water it has to initially have a buoyant force less than force of gravity to make it get submerged. Then when under the water either oxygen releases from submarine or the pressures the water Volume keep the submarine at place to make the F_net = 0