What the sea water temperature during daytime?

Daytime sea water temperatures vary considerably depending on the location (especially latitude) and time of year. The temperatures also vary considerably with depth. Near the poles (Arctic and Antarctic) the surface temperature of sea water is around -4 °C (salt water freezes at a lower temperature than pure water so it is still liquid below 0 °C). Near the equator, surface temperatures may be as high as 36 °C. Note that temperature can vary quite a bit throughout the day. The closer to the surface, the more the temperature varies; on calm days, the temperature at any particular surface location can vary by around 6 °C (10 °F) while it hardly changes at all below 10 meters depth. The temperature of sea water near the ocean floor is around 2-3 °C (except near hydrothermal vents) no matter what time of day. Near hydrothermal vents the temperatures can be 60 °C (140 °F)up to as high as 464 °C (867 °F). Note that at the highest temperatures, the sea water will actually be a supercritical fluid so it wouldn't quite be the liquid phase we normally associate with "water".