Answer: It's just a notion. (It's just an ocean.)
This joke plays on the rhyming sound of the words notion which is another term to refer to an idea and "ocean" which pertains to Pacific - the largest and deepest of the world ocean basins.
Telling a joke is not simple. You have to have good timing and a sense of what your audience is expecting so that you will be able to deliver your joke flawlessly. Check out these tips on how to tell a joke properly like what the professional comedians do, shared by conFITdent.
Also check out these Five Common Mistakes People Make When Telling Jokes from LifeHacker.