What are you even talking about. They had weather stations with mercury thermometers and wrote down temperatures in a logbook.

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.

[1]: https://www.oneoceanexpedition.com/article/checked-150-year-...

Not on the ocean, and not covering even 1/1000% of the coverage we have with satellites on the surface