So the thing with exploration is that an overland journey is just as dangerous as long distance sailing. Both have hazards like weather, plants and animals, terrain/waves, the need for shelter/clothing to protect against the elements, and a need to source food and water locally along the way. Seafaring cultures know where food is and isn't just like how land based explorers know. Expanses of open ocean without fish or rain water is like crossing the desert. Plus the seafaring people know how to build ships that can handle the ocean, so sinking isn't a real danger.

Both had their share of losses as they explored new areas, but guidance by the stars and guidance by landmarks is comparable and they just needed to share knowledge.

I'm not trying to minimize seafaring cutltures, just pointing out that is has different challenges and not necessarily harder challenges than the comparably dangerous overland exploration. Both are very impressive in different ways!