Assuming you want a roughly Earth-like environment on the surface, seas would be immense.
Not necessarily true. Your seas can be the same size as our seas. You just have more continents, the same size as our continents.
And the more sea there is, the more storms a ship is susceptible to meet.
If you have seas much larger than those found on Earth, shipping between continents becomes largely impossible without magic. The size of the vessels is limited by the building materials (in our case, wood - fantasy worlds generally lack the metallurgy to make metal-hulled ships). The crew size of a sailing vessel is determined by the ship size. The crew needs food and fresh water, and you reach a point where the ship cannot carry cargo, because the hold is taken up by the food and water the crew needs for the journey.
So, you'd have to either replace the crew with magic, power the ship by something much more potent than wind, or you need people on board who can Create Food and Water every day. They are the most important person on the ship, as everyone dies without them. End results - the captain of long-haul shipping needs to be able to create food and water for the entire crew, every day. If the captain can't do it, then the spellcaster who can will be able to lead mutiny with alarming success.
This issue goes away if you keep the size of the seas down to something Earth-like.
You get similar issues with super-big continents - their interiors are impassable desert.
Last edited: