The submarine will most likely find data that shows the highest pressure data and the lowest temperature data (unless of course it's near a hydrothermal vent). This is because due to the depth there's a lot more water above it putting a lot of pressure on the sub, and very little sunlight to heat up the water.
The Miller-Urey experiment is important because this was considered as the breakthrough in the study of origin of life, as to where and how exactly life began on earth and that it was possible to form organic molecules from inorganic molecules.
