(1)∫F float = ρ water v row g, the submarine floating out of the sea is smaller than the potential sea surface.

∴ The buoyancy of submarine when it comes out of the sea is less than that under the potential sea;

(2) Buoyancy of submarine:

F float =G row =m row G = 5000×103kg×10n/kg = 5×107n;

∫p =ρGH, the diving depth h increases,

∴ The submarine is under increased water pressure;

∫F float = ρ water V row G, and the volume of seawater discharged by the submarine is unchanged when diving.

The seawater did not change the buoyancy of the submarine.

Therefore, the answer is: (1) less than; (2)5× 107, increasing and unchanged.