A limerick is a piece that follows the AABBA format. That means that lines one and two rhyme with each other, three and four rhyme with each other, and line five rhymes with the first two. So, an example of a limerick about the ocean would be,
“There’s nothing that’s quite like the sea With blue water deep as can be All the shells on the sand In the sun getting tanned Nothing else could be better to me”
because sea, be, and me rhyme, and so do sand and tanned.