I am working on a setting whose low-tech inhabitants are starting to launch long-distance voyages of exploration (preferentially by sea, because travel that way is so much faster than on land). They have outrigger canoes with paddles, and are working on sails.
The limit to how far a team of explorers can go, would seem to be primarily set by supplies of food and fresh water. Along the coast, they can stop to collect water from streams and rivers. For cross-ocean voyages, it's trickier. They don't really have the tech to desalinate seawater, and I get the impression a ship of size X cannot really collect enough rainwater for the crew of a ship of that size, so the water supply would have to be carried on board.
Food is a more subtle question. At first glance it seems to me that they should be able to catch fish on the way. Rationale: a fishing boat can catch more fish in a day than the crew eat, otherwise it would not be profitable! But then, I read about long-distance voyages needing to carry food, so if it was that simple, why would they need to do that?
It is said that the open oceans are deserts; the biological productivity per hectare is low because of lack of availability of mineral nutrients.
Conjecture: profitable fishing of the open ocean (aside from a handful of particularly rich locations) depends on modern technology like sonar to track schools of fish; low-tech fishing techniques like 'two people in an outrigger canoe dangle a net over the side and see what swims into it' are only profitable in the relatively rich waters of the continental shelves.
That would suggest that low-tech explorers can indeed feed themselves along the way by fishing if they are sticking to the coast (where they can also land from time to time to gather berries or suchlike for vitamin C), but when they want to set out across the ocean to find other continents, this will stop working, and they will need to carry food for the voyage.
Is this correct, or am I missing something?