If the "fuel" for nuclear fusion is nuclei of hydrogen, and the earth's oceans are filled with hydrogen atoms all being jostled together, why isn't there a lot of fusion happening in our oceans?

low temperature in oceans

Explanation:

Fission only occurs at extremely high temperatures. For hydrogen nuclei to fuse, they must get very close to each other (achieved ONLY in the presence of a very high temperature), which the nuclei in the oceans can not do because the temperature is too low.