pylon:
The metal contact are probably to detect salt water or sweet water because the two have a different resistance.
Is the density of fresh/salt water significantly different? If so it would make sense that the logger needs to know what type of water it is in, to do the pressure/depth calculation. A resistance sensor could enable it to detect the difference.
On the other hand, I suppose there might be some cost to monitoring the pressure, and the designer might have designed it to only do that when it immersed. I suppose the resistance measurement would tell it that, too.