From the Super Mario Wiki
Lots O'Fish is much like the level Lots O'Jellyfish!, as it is an underwater cavern in Yoshi's Story. Although partly above water, most of it contains dangerous enemies including a Snorkel Snake, Sea Cacti, and Anenomes.
The deep wide ocean would not be an easy swim, but the yoshis dove in. For the first time in their lives, the yoshis swam to the ocean floor. After reaching the shore, the yoshis found their story coming to a close.
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