According to the meaning of the question, the density of seawater is constant during diving.

(1) During diving, the depth is increasing, and the pressure of seawater borne by submersible is increasing from p=ρgh; Therefore, A is right and B is wrong;

(2) During diving, the pressure of seawater on the submersible increases. From p=FS, it can be seen that the seawater pressure increases, so C is wrong;

(3) In the process of diving, the volume of boiling water is unchanged. From F-float = ρ gV, the buoyancy of the submersible is unchanged, that is, the pressure difference of seawater is unchanged, so D is wrong.

So choose a.