Reasons for small temperature difference on cloudy days

1, whether it is cloudy or sunny, the temperature difference between day and night on land is greater than that in the ocean.

2, regardless of land and sea, the temperature difference between day and night is less than that between day and night.

Using the above principle 1 can answer your question: namely

Cloudy oceans have smaller temperature difference than cloudy land.

(Actually, it doesn't matter whether it is cloudy or not. As long as compared with land and sea, the temperature difference between sea and land is smaller than that of land. Unless it's all land or sea, consider the weather)