You could plausibly "need" an arduous overland route, even with well-developed sailing technology and theoretically-possible ocean routes... if those ocean routes were overrun with pirates, swarming with sea monsters, full of icebergs, or fraught with contrary winds and currents. You could "generate" a favorable environment for pirates by making nearby seafaring nations weak (ineffective enforcers) yet wealthy (lucrative prey). Maybe your world's East India Company holds a lock on sea transport there, while an appropriately diplomatic set of land traders might work out routes that skip the institutional legal piracy and graft.

Mapping that? "Here be dragons". Or "exclusive corporate/ guild/ society transport zone". Labels like "the pirate coast", "Thieves' Islands", or a table of required exorbitant tributes. Lots of wreck symbols. Narrow passages with whirlpool indications. Volcanic islands that periodically belch toxic fumes.