That’s not the raw data. The original recordings were made by merchants on parchment. They measured the volume of water in a wooden box, to set the buoyancy for their loads
For the interested, here[1] is an article on an attempt to recreate and verify measurements made during the HMS Challenger expedition in the 1870s.
It was recently done so the full results aren't out, but one aspect they noted was that the traditionally-created hemp rope stretched about 10% so temperatures were taken at slightly deeper depths than expected. This can be used to calibrate the data from HMS Challenger.
I appreciated your comment because more discussion will better help everyone understand the various tranches of surface temperature observations.
I did a quick review, and appreciated the article because they were clear about how their methods different from the recordings. For one using different pressure sensors, and they mentioned the depth differential they measured would lead to variability in the ocean temp readings.
