When you swim through a submerged, level tunnel connecting two bodies of water the pressure in the tunnel depends on $...$

giant sea monsters from an alien planet.

how far you are into the tunnel. The pressure actually changes, decreasing in the middle, because the tunnel walls support the weight of the surrounding fluid.

only the depth of the tunnel. Because the pressure from the fluid above is transmitted through the tunnel according to Pascal's Principle.

how fast you swim through the tunnel.