Fictional oceans are frequently home to leviathans, ancient horrors, mutant abnormalities and other monsters (often of a gargantuan size). Oceans are often depicted as large, mysterious, and unforgiving places. Who is to say if the ship that left port last month did not return because of weather, or because of something else lurking in the deep? Fictional oceans can be a chance to showcase such a fictional creature. If worldbuilders are including such beasts in their seas, however, they may wish to consider the frequency of these elements. Oceans that are home to many sea monsters may have unusual ecosystems. And if these monsters are dangerous to sea-going vessels, the people that sail these waters will have developed methods to avoid or mitigate these dangers.

Storms, like leviathans, are another common element of fictional oceans and seas. Fictional tempests can generate waves the size of buildings, crashing against ship and shore and forming the origin or climax to many narratives that take place on the ocean. Sea storms can bring heroes and castaways to far-away lands, can form the backdrop to thrilling naval battles, or pose an existential threat to characters that have thought themselves greater than the forces of the great ocean.