If you're out in the middle of the ocean?

Question

how much would the high and low differ, on a calm, sunny summer day? Would they even differ at all?

Answer 1

Yes, the high and the low would be different. However, the difference between the two would be very small. Continental regions absorb a lot of long-wave and short-wave solar radiation. Oceans are different. They absorb a lot of long-wave radiation, but reflect a lot of short-wave radiation, most notably blue light. Since water has a high thermal inertia (heats and cools very slowly), the daily range in temperature will be low. This will happen whether the day is sunny or not.