As a fish swims deeper into the ocean, the pressure on its body is:
b.increases
The pressure on the upper surface of the water is the lowest where there is not so much water as of little water pressure, along with a little atmospheric pressure, but if you go deep inside the water the pressure will increases. As on the lower surface of water, the pressure is highest due to heavy water pressure from upwards along with atmospheric pressure. So if a fish swims deeper into an ocean the water will exert more pressure than it is on the upper, to the body of the fish, so the pressure increases.