Short answers, pulled from the story.
Deep ocean water maintains a very uniform temperature of around 0-3 degrees Celsius. It makes up about 90 percent of the total volume of the world's oceans and has a salinity of approximately 3.5 percent, or 35 parts per thousand.
Solar radiation warms the surface, producing temperatures above 30 degrees Celsius in the tropics, while deep water remains near 0-3 degrees Celsius. Strong daytime sunshine combined with low wind speed can create sharp vertical gradients called a diurnal thermocline, with temperature changing rapidly as depth increases.
The primary instrument is the CTD, which stands for conductivity, temperature, and depth, lowered from a ship on a conducting cable. Scientists also use Nansen bottles, bathythermographs, ocean acoustic tomography, and since the 2010s, autonomous gliders and mini-submersibles that carry CTD sensors and transmit data by telemetry.
Weather satellites have been used to measure sea surface temperature since 1967. The first global composites of that satellite data were created in 1970. The Advanced Very High Resolution Radiometer, known as the AVHRR, is one of the most widely used satellite instruments for this purpose.
At the time of the Cambrian Explosion, approximately 538.8 million years ago, sea surface temperatures are estimated to have reached about 60 degrees Celsius. That is above the upper thermal limit of 38 degrees Celsius for modern marine invertebrates.
Warmer water holds less dissolved oxygen than cold water. Increased thermal stratification reduces the mixing of surface oxygen into deeper waters, a process called ocean deoxygenation. Oxygen minimum zones are already expanding worldwide as a result.