Waters high heat capacity protects organisms in the ocean from what?

Heat fluctuations

Explanation:

Water's high heat capacity means that it requires a lot of energy to raise the temperature of a unit volume of water by one degree. It also requires to lose this same amount of energy to lower its temperatures by one degree.

This means that as land temperatures fluctuate a lot from day to night depending on the rate of sunlight hitting the earth, the waters of the oceans remain more or less within very a small range of temperatures. This protects the organism of the oceans from high heat fluctuations that may cause heat shock.