Ocean currents play a major role in climate. Oceans absorb more of the sun’s radiation than the atmosphere, retaining and then distributing heat as the water flows around the planet. This effect is assisted by Earth’s rotation, salinity, and tides (which are in turn affected by the moon). If we have a world without significant oceans, this occurs less and the heat will be concentrated at the equator, leaving the poles more frigid. We can use this to create greater extremes between regions. For Earth-like worlds, we could ignore the ocean’s role unless we’d like to leverage it, but the effect on our work will be subtle and might not be worth the effort to some world builders.
For example, we could invent animals, plants, or species that are found on one coast but not another. Why? Because it happens on Earth. The Gulf Stream that runs along the eastern United States pulls warm water up the coast. The opposite happens on the western coast, where polar water comes south, resulting in much colder waters there. Most seal types prefer colder water and are found west but not east. This is turn means predators like great white sharks are also concentrated where the food is.
If we do this on our invented world, we could have someone from the west coast express disbelief that those around him are about to enter the water on the east coast. Aren’t they afraid to get eaten by a sea creature we’ve invented? They’d likely wonder what he’s talking about, as they don’t exist there. This depends on technological level, as the population of a more modern world like ours is more likely to be aware of differences even if not understanding the reason. In other words, this is arguably more likely in fantasy than in SF.
How do we determine where this might happen? According to NOAA (National Oceanic and Atmospheric Administration), “Major current systems typically flow clockwise in the northern hemisphere and counterclockwise in the southern hemisphere, in circular patterns that often trace the coastlines.” This means that a current is either carrying cold water from the pole toward the equator, or warm water from the equator toward the pole. On Earth, the western coasts are typically colder than the eastern ones. The online, color version of the above picture makes the difference in coastal temperatures easier to see, as the blue arrows are for cold water and are red for warm.
The details of this will depend on how many continents we have and their shape, but we needn’t go overboard and can just decide eastern coastal waters are warmer. A caveat here is that this all depends on the planet rotating in the same direction as Earth. If not, just reverse this.
So how does this affect climate? Warm coastal waters can moderate temperatures in one northern region relative to others, or conversely colder currents cause harsher winters. For example, parts of Norway have mild winters due to the Gulf Stream. We can note this in our files, but avoid explaining it in our stories, as explaining research we’ve done is among the worst, and most obvious, exposition errors.