Q: Why didn’t the girl trust the ocean?
A: Because there was something fishy about it!

Explanation: While there are five named oceans, Arctic, Atlantic, Indian, Pacific and Southern, there is really only one world ocean.  I would recommend having much respect for the ocean both because of its power to give life to the earth, and its dangerous power to take life through storms.  As you already know the ocean has many fish in it.  To trust someone means to have confidence that the person will treat you fairly, be kind, not hurt you.

If something is fishy, that means that there is something suspicious about it.  To be fishy is an idiom. (That reminds me of the old saying, “If something sounds too good to be true, it probably is.”)  Someone offering you $1,000 sounds fishy.  Someone offering to sell you something really cheap sounds fishy, it might be stolen or maybe it doesn’t work.

This joke is funny because it brings together the idea of an ocean full of fish and someone who is fishy.

Back in the 1970s John Denver wrote a song in tribute to Jacques Cousteau who studied the oceans.  I hope you like it.