Complex sentence: Apart from size, the main technical difference between a "submersible" and a "submarine" is that submersibles are not fully autonomous and may rely on a support facility or vessel for replenishment of power and breathing gases.
Split into simpler sentences: Apart from size, the technical difference between a "submersible" and a "submarine" is that submersibles are not totally autonomous. They may rely on a support facility or vessel for replenishment of power and breathing gases.