an ocean, a notion

An ocean or an ocean means one ( an ) sea or large body of salt water ( ocean )--
An ocean is home to many forms of life.
An ocean may describe something related to the sea--
There's an ocean breeze, so it doesn't feel too hot.
They will be sailing to Europe on an ocean liner.

An ocean may also mean, informally,  a vast quantity of something--
"It would take an ocean of calamine lotion..."
We still have an ocean of mail to sort.

A notion or a notion means one ( a ) idea or concept ( notion )--
He had a notion that he'd like to drive there instead of flying.
I have a notion that she dislikes him.

Now that you know that, you can say--
"He sent for these brochures because he had a notion that he'd like to take an ocean cruise."