Say there is a fantasy world where land is extremely rare, with only a few islands specling it's surface. The rest of the world, however, is covered in a single ocean. What would a society be like in a world like this, considering the humans are at our current stage in evolution (not breathing water) and have the technology of the Roman Empire at it's height. Note that this ocean's ecosystem is up for interpretation. My question is, how would these people get resources such as food and potable water,where/what could these people go or build no not drown and how they would acquire the resources necessary. Note that these islands are purely sand based, so any vegetation/food would have to be found at sea. (I understand that it would be impossible for humans to evolve, but becuase this is a fantasy setting I am unconcerned.)
I think you’re likely to have migratory fleets.
Forget solid shelter. Land is scarce. Big floating ‘cities have serious structural issues. Shallow water where you can anchor is (paradoxically) very dangerous, as open water swells will become very fast moving and dangerous breakers.
So go with fleets. These can follow your main food supply (fish), use sunlight distillation to provide a little water and rain for the rest, provide support for each other should one ship run into trouble, avoid major storms and periodically resupply by landing at whatever small archipelagos you may have. They can also swap ships if they meet.
If you have specialist ‘garden boats’ you could even create yourself enough soil to grow plants that can be of use. The main issue will be a sufficient supply of timber to keep the boats in repair and somehow being able to build and repair boats on the move. The former problem can be resolved with a handwavy hard sea plant, the latter can (with a calm sea and enough teamwork) be done by ‘rafting’ many specialised boats to provide a platform for ‘dry’ dock operations.
This won’t eliminate the risk of drowning, sadly, but with many boats supporting each other you can at least reduce the risk, and should you lose part of the fleet the rest can continue.